Turbo / Supercharging - Considerations
Posted: Tue May 06, 2014 7:13 am
I thought I would start to share what I have learnt whilst designing and building my twin charge Alfetta. OK, so I have other threads that go into the detail, but the threads are large and, whilst a good read, are not a concise place to find information. To I will attempt to condense my knowledge here for others to benefit from. My hope, is that we will all do the same, on the areas we are most knowledgeable on. For me, that's engines, or more specifically, the modifications required to significantly increase the output. I will write the post over a week or two. and will try to break it into sections.
If people can post below what information would be really useful to them, I will try to incorporate it.
My background:
14 years as a mechanical engineer. Worked with GM designing body structures for various models. Spent 20 years modifying cars, generally focussed on turbo charging. I am not a mechanic, nor do I profess to be, although, I do my own work and have taught myself (like many others) enough to be dangerous . Any advise or suggestions I give here are personal opinion, backed by research, first hand experience, or gut feel (any combination ). Take my guidance at your own risk is what I am saying. Most of what I suggest may be track only solutions!
Righto, enough of that. The proposed topics (to be adjusted based on peoples interest)
[quote]Turbo or supercharging your engine.[/b]
Why do it?
OK guys (and gals), lets kick it off. I think this is the question that many of us skip over. Maybe you heard a turbo car go buy, or maybe you had a ride in a mates turbo/supercharged car and were hooked from then on. It's important (but not as much fun) to ask yourself why you want to head down the path in the first place. For two reasons, firstly, it's going to cost three times as much as you 'think' it will cost, and secondly, there are many ways to skin this cat, and if you don't get the right advise up front, you will either be unhappy with the end result, or end up putting your car up for sale as an 'unfinished project' and haemorrhaging cash .
I’ll tell you why I got into turbos and superchargers, and then we can talk about what might be a better approach .
I started off in a Fiat 128 3P (actually I finally bought one again...another story). It was to be fair, slow as a wet week, with a max speed of 140Km/hr fully wound out, but I loved it. I wrote that car off, so I bought the biggest engine car I could afford at the time to replace it with my paltry budget. Thank god this was not my first car, or I might never have fallen in love with cars at all. It was a Mitsubishi Sigma with a 2.6 Liter motor. I thought more liters equals more fun right?....how wrong could I be?! That was a hideous asthmatic 4 cylinder that refused to rev. I kicked it to the curb and purchased an Alfetta sedan...I was saved. I then did a bit of research and got interested in rotaries....maybe liters was not everything it was cracked up to be. Well I drove my 12a RX7 around for 12months and loved it....then a 13b turbo motor turned up for sale...hmmm. So I did my first engine swap. I knew stuff all about denotation and I drove that thing around hitting what I thought was the rev limiter, only to find out later the turbo was boost spiking after I put a 3inch exhaust on it which reduced back pressure to the turbo and allowed boost spikes to occur. Unknown to me I was regularly hitting 24+psi. Hats off to that motor, she never blew . I then uprated the turbo on the engine and ended up with over 245 KW at the wheels. The car screamed and was one fast street car. Even then, I wanted more Hp. Lesson number one..... you can NEVER have enough HP.
I have owned about 60 cars since then. You could say I am addicted.
OK, so back to the point, I ‘fell’ into turbo’s. I wanted more Hp and I was fascinated by the way the turbo extracted the Hp. I predominantly used the car as a street car with the occasional track day. These days I modify a car AFTER I work out what the hell I am going to do with it. This has taken me a long time to work out. You don’t modify cars without a purpose. If you do, you will end up driving them on the street, having the occasional drag race, spending most of your time avoiding speed humps and finally having your car impounded, and if you are an ‘enthusiastic’ driver, your licence taken for good measure. 99% of Society does not like the street car modifier....not one bit. I for one got tired of looking over my shoulder.
So here’s the rub, and it goes for all mods. Know the end use of the car. If it’s a road car, try to work within the rules, or at least know the rules....If you want to hit the track, build it to a set of rules, be it drags, circuit, hill climbs etc etc. Just remember, if you don’t build to a class, your car will be worth less at the end of it all, and you will be driving by yourself. The above even applies to those who don’t like to drive their cars so much as show them...there are still rules.
There are many different reasons to modify, no right or wrong way, but there are consequences for choosing one or the other, and if you go into it with your eyes open, the results will not be a surprise to you.
Lets talk about cost. If you head off aimlessly, without a goal in mind, a plan and a budget, by the time you work out what you really wanted, you will have p^$sed a lot of cash up against the wall. Oh....DO NOT get a loan for mods. Mods don’t make you money (almost always) and it sucks selling your car when you get bored or write it off or get it impounded or whatever, and you still have a loan to pay off....happens ALL the time...don’t be that person.
A great mate of mine (ex. Lambo engine builder actually) once said to me, you need a budget. When you get your pay-packet, 40% goes on living expenses, 30% goes to savings (for a house) and 30% goes on toys. It’s darn simple but I like the thinking. Don’t let your toys take over your life. Get yourself a sensible toy budget and stick to it.
So in summary, before you ask ‘how do I turbo a boxer’, first ask yourself ‘what’s my end use of the car, what’s it going to cost, and how long will it take me to save for it? It’s not the fun part, this planning business, but it sure makes for a much more rewarding package.
OK, I now feel like I have given responsible advise, and can get down to the good stuff . How!!
How is power made?
OK, You know you want to boost your engine. You want more power....but what are the variables that lead to more power?? Fuel has been so designed as to carry a lot of energy. The more fuel we can burn the more energy can be released. If your engine can hold together and flow enough, then it's happy days. There are your common street fuels, but of course there are also specialty fuels. To keep things simple we can talk about 98 octane fuel for now. So, to get ignition we need fuel+oxygen+spark. There is a ratio of oxygen to fuel required, it is called the stoichiometric ratio and is 14.7:1. That is, for complete combustion, we need 14.7 parts oxygen to one part fuel (mass..not volume). Righto, so we now know the ratio to mix fuel and oxygen, so it should become obvious that the more air we can shovel into the cylinder, the more fuel we can squirt in and the bigger the bang. So how to we get more in? If we open an intake valve on the engine and then draw the piston down the cylinder, we will pull in a cylinder full of oxygen and fuel, we then want to close the valve before the piston pumps it out of the cylinder (lets not get bogged down in valve tiing for now..maybe later ). This air entered the cylinder at atmospheric pressure, and depending on how high above sea level you are, will decide what pressure this is (that's right....you lose power as you climb a hill). So then if we want more in, all things being equal, we had better push the air/fuel mix in at a pressure higher that atmospheric pressure (force/unit area). Now is a good time to point out that atmospheric pressure at sea level is 14.7 pounds per square inch. Why? Well, if you looked at a one square inch footprint of a column of air to the top of the atmosphere (where psi=0) then it would weigh 14.7 pounds. Interesting when you think about it....or not.. what-evs.
Another thing to keep in mind is the pressure gauge in your car will typically give gauge pressure, that is, it is measuring the difference between atmospheric pressure and the pressure above which your intake is boosted to. So if you measure 14.7psi on you gauge, total pressure is actually 14.7+14.7=29.4psi. This is handy to remember, because superchargers and turbocharger graphs refer to pressure ratio. Pressure ratio is the ratio of absolute pressure to atmospheric pressure. In our example this would be (14.7+14.7)/14.7=2. That means 14.7 psi gauge is doubling the atmospheric pressure. It's a really handy number, intuitively, if someone has an NA engine making 100Hp, and they add a turbo or supercharger that provides 14.7psi gauge, then you would get PR=2, or expect twice as much air and fuel to be shoved in...and expect 2*100Hp = 200Hp. This assumes the rpm at which peak power is made is kept the same and that compression is 100% efficient. Think about this for a bit...I have just told you a very simple way to do a bullshit test on your mates claims :-p.
The main other thing to keep in mind about increasing pressure, is that it is not a 100% efficient process. Typically we are compressing with about 60-80% efficiency. Grab your bike pump and pump up a tyre, you'll notice the pump gets hot. The hotter air is the less dense it is for a given pressure. Take our cylinder moving down with Pressure ratio PR=2. We were shoving two times the air in at PR=2? Well yes, IF we did it at 100% adiabatic efficiency, but we have just said we are not. Make sure you have this clear in your mind....you can have 15psi at 10 degrees C and you can have 15psi at 200 degrees C. The latter will have less kilograms of our desired oxygen in it. Less dense means less oxygen, less oxygen means less fuel, less bang, less Hp...bugger. So the less we heat the air the better, which means the more efficient the turbo the better. But the best we can hope for is 80% efficient compression (roughly), so we still have hotter air, hotter than what? Hotter than the temperature of the day if you have set your intake up properly....hotter than your engine bay temp if you have not . This is where intercooling comes in but we will get to this later.
There is another reason we prefer a cooler charge. It's because fuel will self ignite at a certain temperature and pressure, increasing either past a certain point will see the mix self ignite (detonate). This is why we don’t just run 100psi on street fuel, it’s because the fuel/air mix will explode uncontrollably in the chamber because the spark decides the time is right...usually when the piston is still trying to come up the cylinder. So now you can see that the static pressure ratio of the cylinder must be matched to the boost that you run and the fuel that you intend to use.
In summary: We get more Hp by turbo or supercharging because it squeezes more air into the cylinder. More air allows more fuel to be added. More fuel means a bigger bang, which applies more torque to the crank and in turn the wheel. This increase in pressure comes with increase in temperature, which we want to minimise because it reduces the density and it also takes us closer to the detonation threshold.
What is the difference between Force, Toque and Power?
Above we have been talking about how to increase power for a given rpm. The end goal is to increase power...but where? We know there is a bang in our cylinder, but why does this make power? When the mixture is ignited, we get a sudden increase in pressure in the cylinder as the gas expands, we allow this to happen at the exact right time to maximise the beenfit of this on the piston. This effort on the piston is a force. The force applied, which is the bit we are interested in, is equal to the pressure multiplied by the area of the piston. The bigger the area of the piston for a given pressure, the larger the force....makes sense. We will come back to the fact our pressure is not constant, and therefore the force ever changing post ignition.
OK, so now the bang is converted to a force...next we talk about torque, the use of this force.
Most of you will know that Power=Torque multiplied by rpm. So let’s talk about that a little. Torque is a force applied at a distance from something, this is also called a moment arm. Examples of moment arms are levers...take for example a crow bar. Or try this, hold you are out straight and get your mate to push it near your shoulder, to keep it from moving, you have to apply a force equal to the force he is applying multiplied by the distance he is applying it at. If he then applies the force at your hand, with your arm remaining straight, you will have a great deal more trouble stopping you arm from moving. This is because he has now multiplied with force applied by a larger distance....he is not applying more force.
The explosion in the piston engine applies a force to the piston, in turn the conrod, in turn to the crank. But it does not do this at the center of the crank (hence is is called a crank!) it applies it at some distance (eccentricity) from the center of the shaft. The larger the eccentricity the larger the torque. The distance/eccentricity component of the torque is applied at some angle, but when we calculate torque, force acting at a distance must be determined such that the force is at 90 degrees to the distance used. To do this you first need to determine your coordinate system, that is which direction you call up, and then which way is left and right from up (at 90 deg). Great so now you know that the force applied to the crank needs to be multiplied by the distance from the crank, which is always changing, and it must be done such that the distance is 90 degrees to the force.....which it is not. In fact it is, at top dead center and bottom dead center, but the distance is zero and therefore the torque impacted on the shaft is zero. Everywhere else it is not at 90 degrees. What does this mean to us? It means that the force must be broken into its resultant vectors, you can take any force and brake it into a force triangle.
I don't want this to get bogged down with vectors, so it would suffice to say that when we take our force acting on the crank, and break it into its vectors to get one of the vectors (component of force) acting at 90 degrees to the crank (which we use to multiply by distance to get torque) we are left with a bit of left over force (more than a bit, half way through the power stroke we lose 45% when the moment arm is largest....but the pressure has greatly reduced at this point and so it is of less consequence than it might first appear). So what is this left over force good for? Well nothing, less than nothing. It's not a perfect machine right? This is a mechanical, structural, unavoidable compromise. This left over bit of force is going to try to load our bearings and bend our crank.
Luckily, engines are so arranged that there is an equal and opposite force on another cylinder, an equal distance from the longitudinal midpoint of the crank that balances this force out (actually torque, but I don't want to flip the reference plane on you and loose some people). The point is we lose some torque producing force, and the amount we lose is related to the number of degrees we are through the stroke. You should be able to see why the humble piston engine is actually not very good at converting energy from the fuel into usable power, it's actually only about 30-35% efficient at doing this.....yep, about 65-70% of the energy never makes it to the wheels. How sad.
What is not immediately evident is that as the piston moves down the cylinder, the pressure is reducing, and at the same time this is happening, the lever on our crank is also changing. When the piston is at the top of the cylinder, there is no lever arm. The rods journals are directly in line with the center of the
That is torque, Force*distance about a center. In the case of our engine, torque applied is clearly not consistent for all four strokes for one piston, indeed it is not consistent for one power stroke. As the gas expands, the pressure on the piston resides.
Options?
Lets consider the following options for boosting an engine, Strapping a turbo or a supercharger to a 1.5 boxer should be able to yield 200 crank Hp without trying too hard, so lets face it, either method will put a smile on the dial. Which option you choose comes largely down to how you like your torque delivered. Lets just be clear on our terminology. I will refer to supercharging as a boost adding device, driven mechanically by the crank. Turbochargers are driven by the energy from the exhaust gas exiting the head.
Does everyone here know the how the internals of the turbocharger and supercharger work? Probably but here goes for those who do not;
Supercharger: There are two common types of superchargers, ones that increase the pressure of the air charge inside the supercharger itself, and those that do not. Internal compression superchargers use two rotors that are so shaped that when spun in close proximity to one another, they form a volume of air that is sealed on all sides (mostly). As this volume moves from intake to exit of the supercharger, the volume reduces. If the volume has reduced, but the mass flow of air has not, it stands to reason that the air exits more closely packed together..and so an increase of pressure has occurred.
Turbochargers: these run a common shaft between a compressor wheel and a turbine wheel. The turbine wheel is placed in the exhaust gas flow. The flow has energy in the form of heat and kinetic energy (velocity). The flow is directed into the scroll of the turbine carefully constructed to increase and direct the velocity of the flow onto the blades of the turbine. The turbine responds by spinning up to incredible speeds. This spins the shaft and so to the compressor wheel. The compressor wheel operates in reverse to the turbine wheel. It takes airflow with low kinetic energy and imparts as much as possible to the flow. The airflow exits the compressor wheel and volute faster than the engine can ingest it, with the pressure rising as a consequence. At some point we want to stop the boost increasing either because the engine will detonate, or because the turbo would operate outside is safe/effective range. This regulation is the job of the waste gate. The waste gate bypass's the exhaust gas around the turbine wheel, which means the compressor wheel slows its speed, dropping the boost, the waste gate is then regulated by a pressure signal from the inlet manifold (or other spots depending on desired result).
The main options consist of:
Turbo's:
Single
Twin - parallel
Twin - sequential
Superchargers:
Roots
Screw
Centrifugal (Vortech etc)
Twin charging
Turbo blown supercharger(s)
Supercharger blown turbo charger(s)
to be continued.
How much Hp can I expect? (main variables in the equation)
Preparing the head
Preparing the block
Cooling - charge air (heat is not your friend)
Cooling - coolant
Drivetrain
Things people cock up
The fun bit - an example (twincharge explained) What worked? what did not?
Costs - how much will it cost me??
Useful formulas
Resources (books, links, forums)
If people can post below what information would be really useful to them, I will try to incorporate it.
My background:
14 years as a mechanical engineer. Worked with GM designing body structures for various models. Spent 20 years modifying cars, generally focussed on turbo charging. I am not a mechanic, nor do I profess to be, although, I do my own work and have taught myself (like many others) enough to be dangerous . Any advise or suggestions I give here are personal opinion, backed by research, first hand experience, or gut feel (any combination ). Take my guidance at your own risk is what I am saying. Most of what I suggest may be track only solutions!
Righto, enough of that. The proposed topics (to be adjusted based on peoples interest)
[quote]Turbo or supercharging your engine.[/b]
Why do it?
OK guys (and gals), lets kick it off. I think this is the question that many of us skip over. Maybe you heard a turbo car go buy, or maybe you had a ride in a mates turbo/supercharged car and were hooked from then on. It's important (but not as much fun) to ask yourself why you want to head down the path in the first place. For two reasons, firstly, it's going to cost three times as much as you 'think' it will cost, and secondly, there are many ways to skin this cat, and if you don't get the right advise up front, you will either be unhappy with the end result, or end up putting your car up for sale as an 'unfinished project' and haemorrhaging cash .
I’ll tell you why I got into turbos and superchargers, and then we can talk about what might be a better approach .
I started off in a Fiat 128 3P (actually I finally bought one again...another story). It was to be fair, slow as a wet week, with a max speed of 140Km/hr fully wound out, but I loved it. I wrote that car off, so I bought the biggest engine car I could afford at the time to replace it with my paltry budget. Thank god this was not my first car, or I might never have fallen in love with cars at all. It was a Mitsubishi Sigma with a 2.6 Liter motor. I thought more liters equals more fun right?....how wrong could I be?! That was a hideous asthmatic 4 cylinder that refused to rev. I kicked it to the curb and purchased an Alfetta sedan...I was saved. I then did a bit of research and got interested in rotaries....maybe liters was not everything it was cracked up to be. Well I drove my 12a RX7 around for 12months and loved it....then a 13b turbo motor turned up for sale...hmmm. So I did my first engine swap. I knew stuff all about denotation and I drove that thing around hitting what I thought was the rev limiter, only to find out later the turbo was boost spiking after I put a 3inch exhaust on it which reduced back pressure to the turbo and allowed boost spikes to occur. Unknown to me I was regularly hitting 24+psi. Hats off to that motor, she never blew . I then uprated the turbo on the engine and ended up with over 245 KW at the wheels. The car screamed and was one fast street car. Even then, I wanted more Hp. Lesson number one..... you can NEVER have enough HP.
I have owned about 60 cars since then. You could say I am addicted.
OK, so back to the point, I ‘fell’ into turbo’s. I wanted more Hp and I was fascinated by the way the turbo extracted the Hp. I predominantly used the car as a street car with the occasional track day. These days I modify a car AFTER I work out what the hell I am going to do with it. This has taken me a long time to work out. You don’t modify cars without a purpose. If you do, you will end up driving them on the street, having the occasional drag race, spending most of your time avoiding speed humps and finally having your car impounded, and if you are an ‘enthusiastic’ driver, your licence taken for good measure. 99% of Society does not like the street car modifier....not one bit. I for one got tired of looking over my shoulder.
So here’s the rub, and it goes for all mods. Know the end use of the car. If it’s a road car, try to work within the rules, or at least know the rules....If you want to hit the track, build it to a set of rules, be it drags, circuit, hill climbs etc etc. Just remember, if you don’t build to a class, your car will be worth less at the end of it all, and you will be driving by yourself. The above even applies to those who don’t like to drive their cars so much as show them...there are still rules.
There are many different reasons to modify, no right or wrong way, but there are consequences for choosing one or the other, and if you go into it with your eyes open, the results will not be a surprise to you.
Lets talk about cost. If you head off aimlessly, without a goal in mind, a plan and a budget, by the time you work out what you really wanted, you will have p^$sed a lot of cash up against the wall. Oh....DO NOT get a loan for mods. Mods don’t make you money (almost always) and it sucks selling your car when you get bored or write it off or get it impounded or whatever, and you still have a loan to pay off....happens ALL the time...don’t be that person.
A great mate of mine (ex. Lambo engine builder actually) once said to me, you need a budget. When you get your pay-packet, 40% goes on living expenses, 30% goes to savings (for a house) and 30% goes on toys. It’s darn simple but I like the thinking. Don’t let your toys take over your life. Get yourself a sensible toy budget and stick to it.
So in summary, before you ask ‘how do I turbo a boxer’, first ask yourself ‘what’s my end use of the car, what’s it going to cost, and how long will it take me to save for it? It’s not the fun part, this planning business, but it sure makes for a much more rewarding package.
OK, I now feel like I have given responsible advise, and can get down to the good stuff . How!!
How is power made?
OK, You know you want to boost your engine. You want more power....but what are the variables that lead to more power?? Fuel has been so designed as to carry a lot of energy. The more fuel we can burn the more energy can be released. If your engine can hold together and flow enough, then it's happy days. There are your common street fuels, but of course there are also specialty fuels. To keep things simple we can talk about 98 octane fuel for now. So, to get ignition we need fuel+oxygen+spark. There is a ratio of oxygen to fuel required, it is called the stoichiometric ratio and is 14.7:1. That is, for complete combustion, we need 14.7 parts oxygen to one part fuel (mass..not volume). Righto, so we now know the ratio to mix fuel and oxygen, so it should become obvious that the more air we can shovel into the cylinder, the more fuel we can squirt in and the bigger the bang. So how to we get more in? If we open an intake valve on the engine and then draw the piston down the cylinder, we will pull in a cylinder full of oxygen and fuel, we then want to close the valve before the piston pumps it out of the cylinder (lets not get bogged down in valve tiing for now..maybe later ). This air entered the cylinder at atmospheric pressure, and depending on how high above sea level you are, will decide what pressure this is (that's right....you lose power as you climb a hill). So then if we want more in, all things being equal, we had better push the air/fuel mix in at a pressure higher that atmospheric pressure (force/unit area). Now is a good time to point out that atmospheric pressure at sea level is 14.7 pounds per square inch. Why? Well, if you looked at a one square inch footprint of a column of air to the top of the atmosphere (where psi=0) then it would weigh 14.7 pounds. Interesting when you think about it....or not.. what-evs.
Another thing to keep in mind is the pressure gauge in your car will typically give gauge pressure, that is, it is measuring the difference between atmospheric pressure and the pressure above which your intake is boosted to. So if you measure 14.7psi on you gauge, total pressure is actually 14.7+14.7=29.4psi. This is handy to remember, because superchargers and turbocharger graphs refer to pressure ratio. Pressure ratio is the ratio of absolute pressure to atmospheric pressure. In our example this would be (14.7+14.7)/14.7=2. That means 14.7 psi gauge is doubling the atmospheric pressure. It's a really handy number, intuitively, if someone has an NA engine making 100Hp, and they add a turbo or supercharger that provides 14.7psi gauge, then you would get PR=2, or expect twice as much air and fuel to be shoved in...and expect 2*100Hp = 200Hp. This assumes the rpm at which peak power is made is kept the same and that compression is 100% efficient. Think about this for a bit...I have just told you a very simple way to do a bullshit test on your mates claims :-p.
The main other thing to keep in mind about increasing pressure, is that it is not a 100% efficient process. Typically we are compressing with about 60-80% efficiency. Grab your bike pump and pump up a tyre, you'll notice the pump gets hot. The hotter air is the less dense it is for a given pressure. Take our cylinder moving down with Pressure ratio PR=2. We were shoving two times the air in at PR=2? Well yes, IF we did it at 100% adiabatic efficiency, but we have just said we are not. Make sure you have this clear in your mind....you can have 15psi at 10 degrees C and you can have 15psi at 200 degrees C. The latter will have less kilograms of our desired oxygen in it. Less dense means less oxygen, less oxygen means less fuel, less bang, less Hp...bugger. So the less we heat the air the better, which means the more efficient the turbo the better. But the best we can hope for is 80% efficient compression (roughly), so we still have hotter air, hotter than what? Hotter than the temperature of the day if you have set your intake up properly....hotter than your engine bay temp if you have not . This is where intercooling comes in but we will get to this later.
There is another reason we prefer a cooler charge. It's because fuel will self ignite at a certain temperature and pressure, increasing either past a certain point will see the mix self ignite (detonate). This is why we don’t just run 100psi on street fuel, it’s because the fuel/air mix will explode uncontrollably in the chamber because the spark decides the time is right...usually when the piston is still trying to come up the cylinder. So now you can see that the static pressure ratio of the cylinder must be matched to the boost that you run and the fuel that you intend to use.
In summary: We get more Hp by turbo or supercharging because it squeezes more air into the cylinder. More air allows more fuel to be added. More fuel means a bigger bang, which applies more torque to the crank and in turn the wheel. This increase in pressure comes with increase in temperature, which we want to minimise because it reduces the density and it also takes us closer to the detonation threshold.
What is the difference between Force, Toque and Power?
Above we have been talking about how to increase power for a given rpm. The end goal is to increase power...but where? We know there is a bang in our cylinder, but why does this make power? When the mixture is ignited, we get a sudden increase in pressure in the cylinder as the gas expands, we allow this to happen at the exact right time to maximise the beenfit of this on the piston. This effort on the piston is a force. The force applied, which is the bit we are interested in, is equal to the pressure multiplied by the area of the piston. The bigger the area of the piston for a given pressure, the larger the force....makes sense. We will come back to the fact our pressure is not constant, and therefore the force ever changing post ignition.
OK, so now the bang is converted to a force...next we talk about torque, the use of this force.
Most of you will know that Power=Torque multiplied by rpm. So let’s talk about that a little. Torque is a force applied at a distance from something, this is also called a moment arm. Examples of moment arms are levers...take for example a crow bar. Or try this, hold you are out straight and get your mate to push it near your shoulder, to keep it from moving, you have to apply a force equal to the force he is applying multiplied by the distance he is applying it at. If he then applies the force at your hand, with your arm remaining straight, you will have a great deal more trouble stopping you arm from moving. This is because he has now multiplied with force applied by a larger distance....he is not applying more force.
The explosion in the piston engine applies a force to the piston, in turn the conrod, in turn to the crank. But it does not do this at the center of the crank (hence is is called a crank!) it applies it at some distance (eccentricity) from the center of the shaft. The larger the eccentricity the larger the torque. The distance/eccentricity component of the torque is applied at some angle, but when we calculate torque, force acting at a distance must be determined such that the force is at 90 degrees to the distance used. To do this you first need to determine your coordinate system, that is which direction you call up, and then which way is left and right from up (at 90 deg). Great so now you know that the force applied to the crank needs to be multiplied by the distance from the crank, which is always changing, and it must be done such that the distance is 90 degrees to the force.....which it is not. In fact it is, at top dead center and bottom dead center, but the distance is zero and therefore the torque impacted on the shaft is zero. Everywhere else it is not at 90 degrees. What does this mean to us? It means that the force must be broken into its resultant vectors, you can take any force and brake it into a force triangle.
I don't want this to get bogged down with vectors, so it would suffice to say that when we take our force acting on the crank, and break it into its vectors to get one of the vectors (component of force) acting at 90 degrees to the crank (which we use to multiply by distance to get torque) we are left with a bit of left over force (more than a bit, half way through the power stroke we lose 45% when the moment arm is largest....but the pressure has greatly reduced at this point and so it is of less consequence than it might first appear). So what is this left over force good for? Well nothing, less than nothing. It's not a perfect machine right? This is a mechanical, structural, unavoidable compromise. This left over bit of force is going to try to load our bearings and bend our crank.
Luckily, engines are so arranged that there is an equal and opposite force on another cylinder, an equal distance from the longitudinal midpoint of the crank that balances this force out (actually torque, but I don't want to flip the reference plane on you and loose some people). The point is we lose some torque producing force, and the amount we lose is related to the number of degrees we are through the stroke. You should be able to see why the humble piston engine is actually not very good at converting energy from the fuel into usable power, it's actually only about 30-35% efficient at doing this.....yep, about 65-70% of the energy never makes it to the wheels. How sad.
What is not immediately evident is that as the piston moves down the cylinder, the pressure is reducing, and at the same time this is happening, the lever on our crank is also changing. When the piston is at the top of the cylinder, there is no lever arm. The rods journals are directly in line with the center of the
That is torque, Force*distance about a center. In the case of our engine, torque applied is clearly not consistent for all four strokes for one piston, indeed it is not consistent for one power stroke. As the gas expands, the pressure on the piston resides.
Options?
Lets consider the following options for boosting an engine, Strapping a turbo or a supercharger to a 1.5 boxer should be able to yield 200 crank Hp without trying too hard, so lets face it, either method will put a smile on the dial. Which option you choose comes largely down to how you like your torque delivered. Lets just be clear on our terminology. I will refer to supercharging as a boost adding device, driven mechanically by the crank. Turbochargers are driven by the energy from the exhaust gas exiting the head.
Does everyone here know the how the internals of the turbocharger and supercharger work? Probably but here goes for those who do not;
Supercharger: There are two common types of superchargers, ones that increase the pressure of the air charge inside the supercharger itself, and those that do not. Internal compression superchargers use two rotors that are so shaped that when spun in close proximity to one another, they form a volume of air that is sealed on all sides (mostly). As this volume moves from intake to exit of the supercharger, the volume reduces. If the volume has reduced, but the mass flow of air has not, it stands to reason that the air exits more closely packed together..and so an increase of pressure has occurred.
Turbochargers: these run a common shaft between a compressor wheel and a turbine wheel. The turbine wheel is placed in the exhaust gas flow. The flow has energy in the form of heat and kinetic energy (velocity). The flow is directed into the scroll of the turbine carefully constructed to increase and direct the velocity of the flow onto the blades of the turbine. The turbine responds by spinning up to incredible speeds. This spins the shaft and so to the compressor wheel. The compressor wheel operates in reverse to the turbine wheel. It takes airflow with low kinetic energy and imparts as much as possible to the flow. The airflow exits the compressor wheel and volute faster than the engine can ingest it, with the pressure rising as a consequence. At some point we want to stop the boost increasing either because the engine will detonate, or because the turbo would operate outside is safe/effective range. This regulation is the job of the waste gate. The waste gate bypass's the exhaust gas around the turbine wheel, which means the compressor wheel slows its speed, dropping the boost, the waste gate is then regulated by a pressure signal from the inlet manifold (or other spots depending on desired result).
The main options consist of:
Turbo's:
Single
Twin - parallel
Twin - sequential
Superchargers:
Roots
Screw
Centrifugal (Vortech etc)
Twin charging
Turbo blown supercharger(s)
Supercharger blown turbo charger(s)
to be continued.
How much Hp can I expect? (main variables in the equation)
Preparing the head
Preparing the block
Cooling - charge air (heat is not your friend)
Cooling - coolant
Drivetrain
Things people cock up
The fun bit - an example (twincharge explained) What worked? what did not?
Costs - how much will it cost me??
Useful formulas
Resources (books, links, forums)