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Brake Power 1
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BrianPetersen
Mechanical
Overly simplistic ...
Do you mean "for a single engine, the greater the load on the engine, the higher the efficiency"? Or, translated into more appropriate terminology, "the greater the BMEP of the engine's load setpoint, the higher the efficiency?
Or do you mean "for a range of engines of comparable design but differing size, and at comparable operating conditions in terms of BMEP and operating speed, the bigger engines were more efficient"?
Or do you mean something else, and if so, explain?
Do you mean "for a single engine, the greater the load on the engine, the higher the efficiency"? Or, translated into more appropriate terminology, "the greater the BMEP of the engine's load setpoint, the higher the efficiency?
Or do you mean "for a range of engines of comparable design but differing size, and at comparable operating conditions in terms of BMEP and operating speed, the bigger engines were more efficient"?
Or do you mean something else, and if so, explain?
Compositepro
Chemical
Power output of an engine is proportional to displacement, which is roughly proportional to the cube of its linear dimensions. Energy losses, such as friction and heat loss to the cylinder walls is proportional to surface area, which is roughly proportional to the square of the linear dimensions. So power increases faster than losses when things get bigger. This principle is fairly universal, and is sometimes called "economies of scale".
If you are referring to an engine of constant displacement, the same principle applies but would be stated differently. As you increase power the losses stay relatively constant, so the efficiency increases. An engine at idle has zero efficiency.
If you are referring to an engine of constant displacement, the same principle applies but would be stated differently. As you increase power the losses stay relatively constant, so the efficiency increases. An engine at idle has zero efficiency.
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BrianPetersen
Mechanical
Look up "BSFC map".
Now ... The processes happening inside the cylinders produce heat and pressure that make the engine work. Against that ... some of that heat escapes, some of the pressure leaks past inevitable gaps in valve seats and piston rings, and there's friction between piston rings and cylinder walls and between camshaft lobes and followers and so on, and the coolant pump takes a certain amount of power, and the oil pump takes a certain amount of power, and so on.
Some of those losses are in proportion to the amount of power that you are trying to generate, and some of them are independent of the amount of power you are trying to generate. For example, whatever power it takes to operate the water pump doesn't depend on the power being produced at any given time. Others have complex relationships, e.g. pumping losses.
Simplifying ... If power produced inside the cylinder is 10, and friction is 2, you get out 8 (80%). If power produced inside the cylinder is 5, and friction is 2, you get out 3 (60%). If power produced inside the cylinder is 3, and friction and parasitic losses are still 2, you get out 1 (33.3%). If power produced inside the cylinder is 2, and friction and parasitic losses are still 2, you get out nothing (0%) - that's what is happening when the engine is idling. If power produced inside the cylinder is 1, and friction is still 2, you get negative work ... that's what is happening when you are coasting in gear down a hill ... even more so if you change to a lower gear while doing so.
Now ... The processes happening inside the cylinders produce heat and pressure that make the engine work. Against that ... some of that heat escapes, some of the pressure leaks past inevitable gaps in valve seats and piston rings, and there's friction between piston rings and cylinder walls and between camshaft lobes and followers and so on, and the coolant pump takes a certain amount of power, and the oil pump takes a certain amount of power, and so on.
Some of those losses are in proportion to the amount of power that you are trying to generate, and some of them are independent of the amount of power you are trying to generate. For example, whatever power it takes to operate the water pump doesn't depend on the power being produced at any given time. Others have complex relationships, e.g. pumping losses.
Simplifying ... If power produced inside the cylinder is 10, and friction is 2, you get out 8 (80%). If power produced inside the cylinder is 5, and friction is 2, you get out 3 (60%). If power produced inside the cylinder is 3, and friction and parasitic losses are still 2, you get out 1 (33.3%). If power produced inside the cylinder is 2, and friction and parasitic losses are still 2, you get out nothing (0%) - that's what is happening when the engine is idling. If power produced inside the cylinder is 1, and friction is still 2, you get negative work ... that's what is happening when you are coasting in gear down a hill ... even more so if you change to a lower gear while doing so.
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BrianPetersen
Mechanical
Go find and look at a BSFC map for a random internal combustion engine of the same general concept (4-stroke? 2-stroke? Otto i.e. spark ignition stoichiometric gasoline? Diesel?). Doesn't have to be the exact same thing - just the same general operating principle. (I am ASSuming that this is what we are talking about here ... but ASSumptions can be wrong.)
What you will find is that there is no nice, simple, easy, predictable mathematical relationship here. It is a complex-looking map with the main factors being RPM (generally the horizontal axis) and BMEP i.e. "torque" i.e. "load" (generally the vertical axis).
Some things get better with faster RPM. Heat transfer from gases to surfaces, for example. Some things get worse with faster RPM. The FMEP ("friction mean effective pressure"), for example. The inertial forces involved in changing the directions of motion of pistons and valves go up with the square of RPM, and some component of those forces ends up as friction - between pistons and cylinders, between cam followers and camshafts, whatever.
Then there are confounding effects. If the engine is detonation-limited, generally that's worse at low revs (at high revs, the engine may be spinning fast enough that the self-ignition delay is longer than the time spent with enough pressure and temperature for it to matter). But it might not be. If the engine has lumpy camshafts, the volumetric efficiency at low revs might be so lousy that it won't detonate until it gets into an RPM range where the camshafts actually start working. If it's turbocharged (or centrifugal-supercharged), maybe it won't make enough boost pressure to get the cylinder pressure and temperature into a detonation-prone region until the engine is spinning fast enough for the turbo to make decent boost pressure. If the engine is detonation-limited, perhaps it has to take efficiency-killing countermeasures in certain operating conditions - like delaying the ignition timing, or running rich. That's one reason why the efficiency may drop off right near full load in the affected RPM range.
Lots of engines are thermally limited, too, and have to run rich beyond a certain load setting in order to avoid melting pistons, valves, or catalytic converters.
So, the upshot is that you are asking a very simple question about something that is enormously complex.
What you will find is that there is no nice, simple, easy, predictable mathematical relationship here. It is a complex-looking map with the main factors being RPM (generally the horizontal axis) and BMEP i.e. "torque" i.e. "load" (generally the vertical axis).
Some things get better with faster RPM. Heat transfer from gases to surfaces, for example. Some things get worse with faster RPM. The FMEP ("friction mean effective pressure"), for example. The inertial forces involved in changing the directions of motion of pistons and valves go up with the square of RPM, and some component of those forces ends up as friction - between pistons and cylinders, between cam followers and camshafts, whatever.
Then there are confounding effects. If the engine is detonation-limited, generally that's worse at low revs (at high revs, the engine may be spinning fast enough that the self-ignition delay is longer than the time spent with enough pressure and temperature for it to matter). But it might not be. If the engine has lumpy camshafts, the volumetric efficiency at low revs might be so lousy that it won't detonate until it gets into an RPM range where the camshafts actually start working. If it's turbocharged (or centrifugal-supercharged), maybe it won't make enough boost pressure to get the cylinder pressure and temperature into a detonation-prone region until the engine is spinning fast enough for the turbo to make decent boost pressure. If the engine is detonation-limited, perhaps it has to take efficiency-killing countermeasures in certain operating conditions - like delaying the ignition timing, or running rich. That's one reason why the efficiency may drop off right near full load in the affected RPM range.
Lots of engines are thermally limited, too, and have to run rich beyond a certain load setting in order to avoid melting pistons, valves, or catalytic converters.
So, the upshot is that you are asking a very simple question about something that is enormously complex.
Simplified: 100% thermal efficiency equals zero waste heat which means all of the fuel heating capacity is going into the working fluid (air) which means maximum expansion of this air and corresponding push on the piston and thus maximum power. A 100% efficient engine would have an exhaust temperature equal to the intake temp.
One issue on a gasoline engine is that the engine is throttled by vacuum on the intake side, so as you open the throttle more, you're reducing that vacuum. I've read that diesel engines are a LOT more efficient (that is, use less fuel) when idling, and I assume this is why.
BrianPetersen
Mechanical
Yes, throttling is part of the pumping losses, which tend to be worse at light load. Throttling is like varying the speed of a jogger by varying the amount you strangle the jogger's neck.
Of course, some higher-tech spark-ignition engines address this by fiddling with the cam timing to change the effective length of the intake stroke. If the objective in a part-load intake stroke is to draw in (let's say) a half-cylinder-volume of air, it takes less work to draw in that half-cylinder-volume in the first half of the intake stroke then shut the intake valve and let it expand (vacuum) down to half-atmospheric-pressure, than it is to pull the cylinder down against half-atmospheric-pressure vacuum for the full length of the stroke. Most of the production implementations don't actually work that way but the objective is the same.
I have two such engines in the driveway. The car has Fiat MultiAir variable intake valve duration (done electrohydraulically). The van has separately variable valve timing on both intake and exhaust (DOHC engine).
Of course, some higher-tech spark-ignition engines address this by fiddling with the cam timing to change the effective length of the intake stroke. If the objective in a part-load intake stroke is to draw in (let's say) a half-cylinder-volume of air, it takes less work to draw in that half-cylinder-volume in the first half of the intake stroke then shut the intake valve and let it expand (vacuum) down to half-atmospheric-pressure, than it is to pull the cylinder down against half-atmospheric-pressure vacuum for the full length of the stroke. Most of the production implementations don't actually work that way but the objective is the same.
I have two such engines in the driveway. The car has Fiat MultiAir variable intake valve duration (done electrohydraulically). The van has separately variable valve timing on both intake and exhaust (DOHC engine).
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