Imola
Mechanical
- Jun 8, 2017
- 8
Hello all,
I have a little question about piston engine theory; I was hoping you all could enlighten me. For reference, I am a junior undergrad in mechanical engineering at the University of Illinois (currently studying in Munich-TUM for a semester). I'm a huge a car/motorcycle fan.
I'm curious as to the 'effect' of cylinder count on general torque (and power) output of an engine, keeping displacement and CR equal. Consider two engines, an inline 6 and an inline 4, of equal displacement (we'll say 2 liters). Assume the bore/stroke ratio is the same between them, and the inline 6's cylinders have only been scaled down in size as appropriate.
Here is where my question lies: is the output (brake) torque of the engine always correlated directly with the torque produced by one combustion event in the engine (i.e. the force of one explosion on the con-rod/crankshaft)? This makes sense to me, but if that were true, the 6 cylinder would always have less torque than the equal displacement 4 cylinder (keeping other parameters constant of course). The 6 cylinder would have more combustion events / time though, which would allow it to make relatively more power than the 4 cylinder, perhaps enough to offset the torque advantage of the 4 banger at a given rpm. Also, because of the better balance it would likely rev higher/smoother. BUT, here's an example against this claim that volume of a single cylinder determines (to an extent) the torque output. Take a 3.0 liter F1 engine (v10 or v12). It might make around 260 ft lbs of torque max. My BMW e46 M3 (S54) makes 262 ft lbs of torque in stock form, from 3.2 liters. So the F1 engine is able to make the same torque with much tinier cylinders. How? Is the 'torque' from each combustion event 'added' together to get the brake torque output? I figured that would equate more to the overall power output. I don't see how the output torque could be more than the torque produced by one cylinder (unless its a 'big bang' engine), but I could be thinking about this too simplistically. Is the answer simply that, the torque from the individual cylinders comes in such quick succession that the overall brake torque is greater than that of one explosion?
I always thought of displacement as the main factor for torque output of an engine (at a single rpm, keeping relative head flow characteristics constant) given that it represents the volume of fuel+air that can be crammed into it at atmospheric pressure for a given amount of time. Specific output of an engine is often expressed in terms of torque / liter, especially on an NA engine.
I understand that the total valve surface area will be different for the 2 configurations (more valves / probably better flow with the 6 cylinders) but I didn't really feel like doing that math at the moment. I'd just like to get a little better overall understanding.
Thank you in advance!
I have a little question about piston engine theory; I was hoping you all could enlighten me. For reference, I am a junior undergrad in mechanical engineering at the University of Illinois (currently studying in Munich-TUM for a semester). I'm a huge a car/motorcycle fan.
I'm curious as to the 'effect' of cylinder count on general torque (and power) output of an engine, keeping displacement and CR equal. Consider two engines, an inline 6 and an inline 4, of equal displacement (we'll say 2 liters). Assume the bore/stroke ratio is the same between them, and the inline 6's cylinders have only been scaled down in size as appropriate.
Here is where my question lies: is the output (brake) torque of the engine always correlated directly with the torque produced by one combustion event in the engine (i.e. the force of one explosion on the con-rod/crankshaft)? This makes sense to me, but if that were true, the 6 cylinder would always have less torque than the equal displacement 4 cylinder (keeping other parameters constant of course). The 6 cylinder would have more combustion events / time though, which would allow it to make relatively more power than the 4 cylinder, perhaps enough to offset the torque advantage of the 4 banger at a given rpm. Also, because of the better balance it would likely rev higher/smoother. BUT, here's an example against this claim that volume of a single cylinder determines (to an extent) the torque output. Take a 3.0 liter F1 engine (v10 or v12). It might make around 260 ft lbs of torque max. My BMW e46 M3 (S54) makes 262 ft lbs of torque in stock form, from 3.2 liters. So the F1 engine is able to make the same torque with much tinier cylinders. How? Is the 'torque' from each combustion event 'added' together to get the brake torque output? I figured that would equate more to the overall power output. I don't see how the output torque could be more than the torque produced by one cylinder (unless its a 'big bang' engine), but I could be thinking about this too simplistically. Is the answer simply that, the torque from the individual cylinders comes in such quick succession that the overall brake torque is greater than that of one explosion?
I always thought of displacement as the main factor for torque output of an engine (at a single rpm, keeping relative head flow characteristics constant) given that it represents the volume of fuel+air that can be crammed into it at atmospheric pressure for a given amount of time. Specific output of an engine is often expressed in terms of torque / liter, especially on an NA engine.
I understand that the total valve surface area will be different for the 2 configurations (more valves / probably better flow with the 6 cylinders) but I didn't really feel like doing that math at the moment. I'd just like to get a little better overall understanding.
Thank you in advance!
