Why r big engine cars so inefficient @driving around slowly?
Discussion
After the great response to my tuning question, I thought I might try this one which has not had an answer on the Porsche section yet.
Why are big engine cars so inefficient at driving around slowly? I can understand that when you are accelerating and speeding like mad, the mpg will be low because you need a lot of energy to go fast. However, when you are following a Ford Fiesta around town in a 911, why is the Fiesta getting twice the mpg achieved by the 911. The weight isn't all that different, so where has all the energy gone in the 911?!
Why are big engine cars so inefficient at driving around slowly? I can understand that when you are accelerating and speeding like mad, the mpg will be low because you need a lot of energy to go fast. However, when you are following a Ford Fiesta around town in a 911, why is the Fiesta getting twice the mpg achieved by the 911. The weight isn't all that different, so where has all the energy gone in the 911?!
quyen said:
After the great response to my tuning question, I thought I might try this one which has not had an answer on the Porsche section yet.
Why are big engine cars so inefficient at driving around slowly? I can understand that when you are accelerating and speeding like mad, the mpg will be low because you need a lot of energy to go fast. However, when you are following a Ford Fiesta around town in a 911, why is the Fiesta getting twice the mpg achieved by the 911. The weight isn't all that different, so where has all the energy gone in the 911?!
Nope, you've got me there.Why are big engine cars so inefficient at driving around slowly? I can understand that when you are accelerating and speeding like mad, the mpg will be low because you need a lot of energy to go fast. However, when you are following a Ford Fiesta around town in a 911, why is the Fiesta getting twice the mpg achieved by the 911. The weight isn't all that different, so where has all the energy gone in the 911?!
PS: Your tunning question
I dunno, quite an interesting question when you think about it... maybe something to do with the thermal efficiency of a small fire in a small pot vs. a small fire in a big pot?
eta, thinking about it - doesn't the vast proportion of the waste in an engine come out as heat? So in a big engine throttled back to 20hp or something just cruising at 30mph, theres much more surface area in the cylinder to loose heat out of. So you've got to burn a bit more mixture to compensate for the heat lost out of the walls compared to a tiny engine with a smaller surface area in the cylinder.
I think that would be the main reason, friction, pumping losses etc would also be a part of it...
Thats my theory anyway!
eta, thinking about it - doesn't the vast proportion of the waste in an engine come out as heat? So in a big engine throttled back to 20hp or something just cruising at 30mph, theres much more surface area in the cylinder to loose heat out of. So you've got to burn a bit more mixture to compensate for the heat lost out of the walls compared to a tiny engine with a smaller surface area in the cylinder.
I think that would be the main reason, friction, pumping losses etc would also be a part of it...
Thats my theory anyway!
Edited by buggalugs on Saturday 26th April 15:18
eliot said:
Friction.
and (to a much smaller extent) increased pumping losses from running a larger engine at lower manifold pressures. In theory a bigger engine can be just as economical as a smaller one if you run it at lower revs i.e. twice the capacity half the revs gives the same power and roughly equal fuel economy, problem with this theory is that engines don't generally like being run very slowly.>Buggalugs said:
>I dunno, quite an interesting question when you think about it... maybe
>something to do with the thermal efficiency of a small fire in a small pot
>vs. a small fire in a big pot?
Buggalugs, I think you may be closest to the true.
I found the following on wikipedia:
http://en.wikipedia.org/wiki/Engine_efficiency
"It should be noted that at lower power outputs, the effective compression ratio is less than when the engine is operating at full power, due to the simple fact that the incoming fuel-air mixture is being restricted. Thus the effective engine efficiency will be less than when the engine is producing its maximum rated power. One solution to this fact is to shift the load in a multi-cylinder engine from some of the cylinders (by deactivating them) to the remaining cylinders so that they may operate under higher individual loads and with correspondingly higher effective compression ratios. This technique is known as variable displacement."
The variable displacement solution is here:
http://en.wikipedia.org/wiki/Variable_displacement
With the price of fuel these days, I wish I had variable displacement in my car. :-(
>I dunno, quite an interesting question when you think about it... maybe
>something to do with the thermal efficiency of a small fire in a small pot
>vs. a small fire in a big pot?
Buggalugs, I think you may be closest to the true.
I found the following on wikipedia:
http://en.wikipedia.org/wiki/Engine_efficiency
"It should be noted that at lower power outputs, the effective compression ratio is less than when the engine is operating at full power, due to the simple fact that the incoming fuel-air mixture is being restricted. Thus the effective engine efficiency will be less than when the engine is producing its maximum rated power. One solution to this fact is to shift the load in a multi-cylinder engine from some of the cylinders (by deactivating them) to the remaining cylinders so that they may operate under higher individual loads and with correspondingly higher effective compression ratios. This technique is known as variable displacement."
The variable displacement solution is here:
http://en.wikipedia.org/wiki/Variable_displacement
With the price of fuel these days, I wish I had variable displacement in my car. :-(
It's all about thermal efficiency as some have mentioned. Most of the energy is lost as heat as only around 35% of the energy released by the fuel goes into forward motion. The greater the surface area of the combustion chamber the less efficient it is at converting the energy released to piston pushing force before the energy is lost as heat into the piston crown, cylinder wall and head. Remember the ratio of area to volume is a squared one. I reckon the fires in big tins/small tins analogy is very good. Never heard that one before.
To save anyone the trouble, yes I know you can have more cylinders of smaller area to make a huge engine, but more cylinders = more surface area, the relationship between cubic capacity and total area remains constant (I think) regardless of how you split it up into cylinders.
jon
To save anyone the trouble, yes I know you can have more cylinders of smaller area to make a huge engine, but more cylinders = more surface area, the relationship between cubic capacity and total area remains constant (I think) regardless of how you split it up into cylinders.
jon
As many have mentioned.
Loses due to Friction, more parts all rubbing and using up energy.
Driving slowly, I guess you're talking around town, in which case you will be spending a lot of time idling therefore you will be consuming a lot more fuel just to idle.
BSFC will also be lower as due to the greater number of cylinders each one will have to work less. This is mainly due to the closed throttles and as previously mentioned the energy wasted pumping air backwards and forwards. An engine is it's most efficent at WoT.
Also a bigger engine tends to be fitted to a bigger or heavyier car and therefore will have more mass to get moving (not keep moving which is easier).
Loses due to Friction, more parts all rubbing and using up energy.
Driving slowly, I guess you're talking around town, in which case you will be spending a lot of time idling therefore you will be consuming a lot more fuel just to idle.
BSFC will also be lower as due to the greater number of cylinders each one will have to work less. This is mainly due to the closed throttles and as previously mentioned the energy wasted pumping air backwards and forwards. An engine is it's most efficent at WoT.
Also a bigger engine tends to be fitted to a bigger or heavyier car and therefore will have more mass to get moving (not keep moving which is easier).
Just to be contrary, I'm going to disagree with a lot of that and say it's down to VOLUMETRIC efficiency. I still feel that most of what has been said is true but I don't believe it's the principal reason.
Petrol engines need to work (as hs been said) at about 14:1 air to fuel ratio. Under hard acceleration they need to run a but richer and on part throttle they can run a bit weaker - but not much without misfiring (or worse, detonating). To restrict the power of a petrol engine, you need to restrict the amount of air as well as the amount of fuel to keep within that narrow band, so they have a throttle plate which obstructs the air going into the cylinders. This means that the cylinders only partially fill (very little volume goes in) - hence the term "volumetric" efficiency. I therefore disagree with the Wikipedia entry because if anything, the engine's cylinders will fill BETTER when the throttle is wide-open than when it is closed! Obviously, there is a trade-off. At max RPM, even with the throttle fully open, there will be a limit to the amount of air that can be sucked into the cylinder during the short time the inlet valve is open. (Hence the popularity of forced induction)!
Diesels are MUCH better in this respect. They can run at air : fuel ratios of 50 or 60 to 1 without damaging themselves so they can usually get away without a throttle plate. To restrict the power of a diesel, just give it less fuel but the air volume going into the cylinders can remain the same.
At low (town) speed driving, the big petrol engine will be working at TINY throttle openings. All the cylinders will be very poorly filled on each revolution - hence the lousy fuel economy. I believe that research (largely in the US), has focused on partial cylinder shut-down so that some cylinders get no fuel at all (or spark) and the remaining ones can then afford to work harder for the same overall power output so they are being filled more efficiently.
A small petrol engine running under the same conditions will have it's throttle open wider than the big car for the same power output so it's cylinders are filling more efficiently - hence the better economy under those conditions.
A diesel engine won't have the same problem - which is why they absolutely slaughter petrol engines under part-throttle conditions.
I have an old 3 litre petrol car and a 2 litre modern diesel MPV. In town, the MPV can manage 40 to the gallon and the petrol car (despite being much lighter) barely manages 20. On the motorway at 85-90, there is much less to choose between them - petrol car will get close to 30 MPG, the diesel MPV will only do about 33 - 34. It's not a brilliant example because of the MPV's bigger frontal area but it's a general indicator.
The trick is to match the engine size to the conditions. Working at part throttle, the petrol engine is very inefficient. Working at full throttle and full RPM, it is also inefficient because instead of the throttle plate restricting the volume of air going into it, the fundamental limitations of the inlet tract are doing so instead. Best economy tends to be around peak torque where the cylinders are all filling as much as possible. (typically lower RPM and larger throttle openings).
Petrol engines need to work (as hs been said) at about 14:1 air to fuel ratio. Under hard acceleration they need to run a but richer and on part throttle they can run a bit weaker - but not much without misfiring (or worse, detonating). To restrict the power of a petrol engine, you need to restrict the amount of air as well as the amount of fuel to keep within that narrow band, so they have a throttle plate which obstructs the air going into the cylinders. This means that the cylinders only partially fill (very little volume goes in) - hence the term "volumetric" efficiency. I therefore disagree with the Wikipedia entry because if anything, the engine's cylinders will fill BETTER when the throttle is wide-open than when it is closed! Obviously, there is a trade-off. At max RPM, even with the throttle fully open, there will be a limit to the amount of air that can be sucked into the cylinder during the short time the inlet valve is open. (Hence the popularity of forced induction)!
Diesels are MUCH better in this respect. They can run at air : fuel ratios of 50 or 60 to 1 without damaging themselves so they can usually get away without a throttle plate. To restrict the power of a diesel, just give it less fuel but the air volume going into the cylinders can remain the same.
At low (town) speed driving, the big petrol engine will be working at TINY throttle openings. All the cylinders will be very poorly filled on each revolution - hence the lousy fuel economy. I believe that research (largely in the US), has focused on partial cylinder shut-down so that some cylinders get no fuel at all (or spark) and the remaining ones can then afford to work harder for the same overall power output so they are being filled more efficiently.
A small petrol engine running under the same conditions will have it's throttle open wider than the big car for the same power output so it's cylinders are filling more efficiently - hence the better economy under those conditions.
A diesel engine won't have the same problem - which is why they absolutely slaughter petrol engines under part-throttle conditions.
I have an old 3 litre petrol car and a 2 litre modern diesel MPV. In town, the MPV can manage 40 to the gallon and the petrol car (despite being much lighter) barely manages 20. On the motorway at 85-90, there is much less to choose between them - petrol car will get close to 30 MPG, the diesel MPV will only do about 33 - 34. It's not a brilliant example because of the MPV's bigger frontal area but it's a general indicator.
The trick is to match the engine size to the conditions. Working at part throttle, the petrol engine is very inefficient. Working at full throttle and full RPM, it is also inefficient because instead of the throttle plate restricting the volume of air going into it, the fundamental limitations of the inlet tract are doing so instead. Best economy tends to be around peak torque where the cylinders are all filling as much as possible. (typically lower RPM and larger throttle openings).
Avocet said:
Just to be contrary, I'm going to disagree with a lot of that and say it's down to VOLUMETRIC efficiency.
That certainly plays a part, but friction losses play a part too (a much larger part under most conditions). Friction and pumping losses can both be reduced by running a small engine with more throttle, or running the same engine at lower speed with more throttle.Yep and the highest VolEff is usually acheived at peak torque and power where the engine has been tuned to have it's greatest output. The greatest gain will be had when at full throttle.
A part throttle condition will be less efficient. You could go into fuelling for protecting components and things like that but that will bring in a whole load more variables.
Diesels are more efficient as like you say that can operate at varying AFR's yet mainly they have no throttle and improvements in output are soley controlled by the amount of fuel injected.
A part throttle condition will be less efficient. You could go into fuelling for protecting components and things like that but that will bring in a whole load more variables.
Diesels are more efficient as like you say that can operate at varying AFR's yet mainly they have no throttle and improvements in output are soley controlled by the amount of fuel injected.
GreenV8S said:
Avocet said:
Just to be contrary, I'm going to disagree with a lot of that and say it's down to VOLUMETRIC efficiency.
That certainly plays a part, but friction losses play a part too (a much larger part under most conditions). Friction and pumping losses can both be reduced by running a small engine with more throttle, or running the same engine at lower speed with more throttle.
My average speed is the same in both methods, and hence my friction losses would be the same. However I can up my economy from ~20mpg to ~25mpg by playing with the throttle. That is on 15 mile cross country run with lots of stops and starts and speed limit changes, staring at the back of a bland silver repmobile and nothing more interesting to do than play with the throttle
Edited by funwithrevs on Monday 28th April 10:29
Avocet said:
To restrict the power of a petrol engine, you need to restrict the amount of air as well as the amount of fuel to keep within that narrow band, so they have a throttle plate which obstructs the air going into the cylinders. This means that the cylinders only partially fill (very little volume goes in) - hence the term "volumetric" efficiency.
Ermm, the volume of the cyclinder is constant so I think you mean very little air mass (molecules) goes in. Less air mass means less air density (mass/volume), less air pressure to start with and less pressure when fully compressed. From my simple understanding of car engines, less pressure means lower efficiency. Diesels engines are more efficient because they operate at higher pressures. Can anyone explain why higher operating pressures leads to greater effiency?
Avocet said:
I therefore disagree with the Wikipedia entry because if anything, the engine's cylinders will fill BETTER when the throttle is wide-open than when it is closed!
I will have to re-read the Wikipedia entry but I thought they were saying exactly what you are saying.Avocet said:
Diesels are MUCH better in this respect. They can run at air : fuel ratios of 50 or 60 to 1 without damaging themselves so they can usually get away without a throttle plate. To restrict the power of a diesel, just give it less fuel but the air volume going into the cylinders can remain the same.
My understanding from Wikipedia is that lean burning in a petrol engine leads to unacceptable NOx productions. Why don't diesels engines have this same problem? quyen said:
Ermm, the volume of the cyclinder is constant so I think you mean very little air mass (molecules) goes in. Less air mass means less air density (mass/volume), less air pressure to start with and less pressure when fully compressed. From my simple understanding of car engines, less pressure means lower efficiency. Diesels engines are more efficient because they operate at higher pressures.
Can anyone explain why higher operating pressures leads to greater effiency?
Yes, that's what I meant - isn't that what I said?Can anyone explain why higher operating pressures leads to greater effiency?
Can't explain the pressure thing without getting some old text books out! I think it's something to do with the "Carnot" cycle of pressure vs. volume. I think, (but am not sure!) that the useful "work" that can be had from an engine is related to the difference between starting pressure (before the spark ignites the mixture) and the finishing pressure (as the piston reaches BDC) so the greater we can make that difference, the more "efficient" the engine will be. Unfortunately, there comes a point when if you compress petrol and air too much it explodes anyway - which limits what can be done at the top end of the cycle and at the bottom end, we'd never be able to get to zero pressure in the time available.
It's true that running a petrol engine weak produces more NOx but I was talking about much weaker mixtures than that - the sort that would cause engine damage in a petrol engine don't seem to bother diesels.
Avocet said:
Just to be contrary, I'm going to disagree with a lot of that and say it's down to VOLUMETRIC efficiency. I still feel that most of what has been said is true but I don't believe it's the principal reason.
Running at part throttle will certainly effect volumetric efficiency, but this does not actually explain why the engine will use comparatively more fuel (i.e. higher BSFC). The answer (already stated) is the low volumetric efficiency gives a lower dynamic compression ratio, and the thermal efficiency of the Otto cycle is directly related to CR. Therefore low volumetric efficiency = low thermal efficiency (given a fixed CR optimised for high volumetric efficiency). This is also why turbocharged engines tend to have poor fuel economy, as most of the time they are not actually producing boost, hence low CR.Larger capacity cylinders also tend to lose more heat from the combustion (despite a more favourable surface/volume ratio) to the pistons, cylinders and combustion chamber due to the finite burn time of the mixture. More cylinders also give higher frictional losses.
In terms of a 911 vs Fiesta, the Porsche will have wide, sticky tyres which increases rolling resistance, and the car will be somewhat heavier, all adding up to poor mpg in typical urban useage.
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