daft questions about engine efficiency
Discussion
I like to think that I've got a reasonable grasp of the workings of engines but there is a few things that I don't quite understand.
In an ideal world the flowrate (of air) through an engine would be as high as possible for a given rpm. However the valves are the main source of resistance, hence things like 4 valves per cylinder.
However someone said in another thread (to do with head systems on TVR Rover v8's) that if the airflow is too slow then the engine will become less efficient. Surely having to accelerate the air up will cause losses and decrease your air flowrate.
I would have thought that the most efficient engine would have the most free flowing intake system without any losses.
So what is it I'm missing?
Andy
In an ideal world the flowrate (of air) through an engine would be as high as possible for a given rpm. However the valves are the main source of resistance, hence things like 4 valves per cylinder.
However someone said in another thread (to do with head systems on TVR Rover v8's) that if the airflow is too slow then the engine will become less efficient. Surely having to accelerate the air up will cause losses and decrease your air flowrate.
I would have thought that the most efficient engine would have the most free flowing intake system without any losses.
So what is it I'm missing?
Andy
If the air speed is too low then fuel can form droplets in the manifold. A bit like condensation on a window. This can cause problems. Another issue is that the volumes and lengths of intake and exhaust systems are tuned for an engine. The right lengths and volumes give the best power outputs but only at one engine speed. This is the same idea as getting the right notes out of a brass band trombone by moving the sliding part in and out. This is why some engines have variable geometry intakes to spread the power over a wider range of engine speeds.
Big manifolds give big power at high speeds but might not be good anywhere else in the rev range.
Big manifolds give big power at high speeds but might not be good anywhere else in the rev range.
Fuel drop out is only a small factor, with the advent of GDI engines, it's not even much of a problem anymore.
However, the energy of the gasses moving both in and out of the engine are important. The higher the speed, the more energy the gas has (e=0.5mv^2) and the more difficult it is to reverse to stop the flow. With a high port speed, the inlet valves opening period can be extended to beyond BTDC, and the energy in the gasses will ensure that they continue to move into the cylinder. A simmilar situation occurs with exhaust gasses. Using these techniques mean a naturaly aspirated engine can actually have a volumetric efficiency of greater than 100%.
Post speed beomces more of a problem at low engine speeds. Large ports and a high duration/overlap cam, designed for ultimate power means the engine suffers badly at low RPM. By carefully shaping the ports to retain the flow, but at a higher port speed gives the engine more torque at low RPM and allows the cam to start working at a lower RPM.
However, the energy of the gasses moving both in and out of the engine are important. The higher the speed, the more energy the gas has (e=0.5mv^2) and the more difficult it is to reverse to stop the flow. With a high port speed, the inlet valves opening period can be extended to beyond BTDC, and the energy in the gasses will ensure that they continue to move into the cylinder. A simmilar situation occurs with exhaust gasses. Using these techniques mean a naturaly aspirated engine can actually have a volumetric efficiency of greater than 100%.
Post speed beomces more of a problem at low engine speeds. Large ports and a high duration/overlap cam, designed for ultimate power means the engine suffers badly at low RPM. By carefully shaping the ports to retain the flow, but at a higher port speed gives the engine more torque at low RPM and allows the cam to start working at a lower RPM.
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