When is an engine at its most efficient?
Discussion
Here's the scenario: accelerating away from standstill (toll booth?) on a flat motorway. I want to reach 70mph, but I want to get there using the least amount of fuel possible. Talking scientifically, this will take x amount of energy - to accelerate the mass of my car to the target speed.
I've read (somewhere, can't remember where) that an engine is at its most efficient at full throttle - so that is, a greater percentage of the energy in the fuel will end up being converted into kinetic energy at full throttle that part throttle.
Does it follow therefore, that I should put my foot hard down, attain 70mph in the shortest time and ease back? It seems counter intuitive, but if accelerating gently is meaning the engine is running less efficiently, and I only need to put x amount of energy into the car, then more fuel will be used (probably a much higher "instant readout mpg", but over the longer acceleration period).
Any thoughts?
I've read (somewhere, can't remember where) that an engine is at its most efficient at full throttle - so that is, a greater percentage of the energy in the fuel will end up being converted into kinetic energy at full throttle that part throttle.
Does it follow therefore, that I should put my foot hard down, attain 70mph in the shortest time and ease back? It seems counter intuitive, but if accelerating gently is meaning the engine is running less efficiently, and I only need to put x amount of energy into the car, then more fuel will be used (probably a much higher "instant readout mpg", but over the longer acceleration period).
Any thoughts?
Putting it simply the reason for the idea that full throttle is best is due to a petrol engine only getting its cylinder full of air at full throttle, so you can then reach a decent compression ratio when you burn the fuel air mix. ie. Full throttle gives you say 9.5:1 CR whilst half throttle might only give you 5:1 CR. You need to get the compression ration as high as possible to maximise the heat generated and therefore power from the amount of fuel available, so anything lower than peak CR is less efficient. This is why diesel engine are so much better on MPG as they always run at maximum compression ratio. So in your test case you need to change up gear as soon as you can, and use a wider throttle opening at lower RPM (within sensible powerbands) to get the best efficiency from the engine. Keeping the RPM lower also reduces your friction and pumping losses. There are lots of other factors involved, but this is the basic idea.
blitzracing said:
Putting it simply the reason for the idea that full throttle is best is due to a petrol engine only getting its cylinder full of air at full throttle, so you can then reach a decent compression ratio when you burn the fuel air mix. ie. Full throttle gives you say 9.5:1 CR whilst half throttle might only give you 5:1 CR. You need to get the compression ration as high as possible to maximise the heat generated and therefore power from the amount of fuel available, so anything lower than peak CR is less efficient. This is why diesel engine are so much better on MPG as they always run at maximum compression ratio. So in your test case you need to change up gear as soon as you can, and use a wider throttle opening at lower RPM (within sensible powerbands) to get the best efficiency from the engine. Keeping the RPM lower also reduces your friction and pumping losses. There are lots of other factors involved, but this is the basic idea.
Very interesting.So this technique of using WOT at sensible RPM levels will be effective in a petrol engine, but possibly counter productive in a modern diesel engine which can get a good efficient fuel burn even at part throttle?
I sense some reading up coming on, I must admit despite now owning a modern CR diesel engined car, anything non-petrol is still witchcraft as far as my knowledge is concerned!
timbob said:
Very interesting.
So this technique of using WOT at sensible RPM levels will be effective in a petrol engine, but possibly counter productive in a modern diesel engine which can get a good efficient fuel burn even at part throttle?
I sense some reading up coming on, I must admit despite now owning a modern CR diesel engined car, anything non-petrol is still witchcraft as far as my knowledge is concerned!
Diesels (new and otherwise) are at WOT all the time. "Throttling" is achieved by varying the fuel input. So a diesel is taking advantage of the highest possible CR at all times.So this technique of using WOT at sensible RPM levels will be effective in a petrol engine, but possibly counter productive in a modern diesel engine which can get a good efficient fuel burn even at part throttle?
I sense some reading up coming on, I must admit despite now owning a modern CR diesel engined car, anything non-petrol is still witchcraft as far as my knowledge is concerned!
The problem with using WOT in a petrol engine is the fuel mixture goes open loop to a richer mixture than at cruise and economy falls. Therefore for best economy during acceleration change to the highest useable gear as soon as possible and use 3/4 throttle to maximise cylinder filling while still staying at stoichiometric A/F ratio.
This has been done to death in those fuel economy competitions where it's been long established that "firm", but not full, throttle application in a high gear is better than light throttle in lower gears.
This has been done to death in those fuel economy competitions where it's been long established that "firm", but not full, throttle application in a high gear is better than light throttle in lower gears.
Pumaracing said:
The problem with using WOT in a petrol engine is the fuel mixture goes open loop to a richer mixture than at cruise and economy falls. Therefore for best economy during acceleration change to the highest useable gear as soon as possible and use 3/4 throttle to maximise cylinder filling while still staying at stoichiometric A/F ratio.
Why is running at stoich AFR most efficient from a fuel comsumption perspective; you can safely run leaner on most production cars, where ignition curves can also be advanced on the fly. If you use a wide band lambda, and you have mapped for WOT, does this necesserily mean you will end up running open loop?
Is the efficieny equation also not a function of the BMEP of the engine and the revs at which peak torque is delivered (in addition to maximising cylinder filling and an optimum AFR)?!
fergus said:
Why is running at stoich AFR most efficient from a fuel comsumption perspective; you can safely run leaner on most production cars, where ignition curves can also be advanced on the fly.
Presumably because stoich = most efficient. You can run leaner, but the mix won't release as much energy.fergus said:
Is the efficieny equation also not a function of the BMEP of the engine and the revs at which peak torque is delivered (in addition to maximising cylinder filling and an optimum AFR)?!
In terms of overall fuel efficiency, I should think so, along with vehicle weight, drag coefficient, frictional losses through the drivetrain, etc. The entire car behaves as a system.Stoich isn't the most efficient A/F ratio but it's what anything modern with a cat is limited to so ain't much anyone can do about it and still stay legal.
On my old pre-cat Fiesta XR2i I could wind the fuel pressure regulator screw in with an allen key to weaken the mixture and get incredible fuel consumption at the expense of a bit of power. For a car that averaged 34 mpg over 10k miles and could normally only just squeeze 40 mpg on a motorway run I once got 52 mpg over 200 miles with a very weak mixture and ultra careful driving.
By contrast there's not a damn thing I can do to improve mpg on my Focus and even driving slowly makes no great difference. Too heavy and too much tyre drag with the 205 section tyres compared to older lighter cars on skinnier rubber. 30 mpg overall and best of about 37/38 on a run. Well under 30 mpg around town or on short journeys.
Modern cars might be good on emissions but they're less fuel efficient than many older ones. A nice light car without all the safety and emissions crap that adds several hundred kilos and with an SU carb will do more mpg than most modern fuel injection ones.
On my old pre-cat Fiesta XR2i I could wind the fuel pressure regulator screw in with an allen key to weaken the mixture and get incredible fuel consumption at the expense of a bit of power. For a car that averaged 34 mpg over 10k miles and could normally only just squeeze 40 mpg on a motorway run I once got 52 mpg over 200 miles with a very weak mixture and ultra careful driving.
By contrast there's not a damn thing I can do to improve mpg on my Focus and even driving slowly makes no great difference. Too heavy and too much tyre drag with the 205 section tyres compared to older lighter cars on skinnier rubber. 30 mpg overall and best of about 37/38 on a run. Well under 30 mpg around town or on short journeys.
Modern cars might be good on emissions but they're less fuel efficient than many older ones. A nice light car without all the safety and emissions crap that adds several hundred kilos and with an SU carb will do more mpg than most modern fuel injection ones.
Interesting... I'd just assumed that stoichioemetry = max efficiency. I guess that's why I'm in IT and not mechanical engineering 
Just found this on the principle of lean burn engines...
http://en.wikipedia.org/wiki/Lean_burn
Just found this on the principle of lean burn engines...
http://en.wikipedia.org/wiki/Lean_burn
nope!
Fuel efficiency = make the required power with the least fuel burnt
Fuel economy = burn the least fuel
Not the same what so ever..........
I.e for a roadload of say 10kW, you want to run at say a load site that gives you the best BSFC, and say 6th gear might be 10% better BSFC than 5th gear for that roadspeed/load, and as such, burn 10% less fuel
But if you drive in such a fashion to reduce the mean roadload to 5kW, you could operate at a load point with very poor efficiency and still burn less fuel.
Fuel efficiency = make the required power with the least fuel burnt
Fuel economy = burn the least fuel
Not the same what so ever..........
I.e for a roadload of say 10kW, you want to run at say a load site that gives you the best BSFC, and say 6th gear might be 10% better BSFC than 5th gear for that roadspeed/load, and as such, burn 10% less fuel
But if you drive in such a fashion to reduce the mean roadload to 5kW, you could operate at a load point with very poor efficiency and still burn less fuel.
I seem to remember that back in the 80s or 90s VW did an experiment on this subject. They set out a measured length of road, then the test car had to enter the length of road at a defined low speed and exit at a defined high speed. Within that length of road the car could accelerate gently or at full blast or at any rate in between. They even had the car accelerate full blast to a speed higher than required and then reduce to the defined speed before exiting the measured length of road. They found that the technique that used least fuel was the full blast acceleration, in second place was the technique that over-speeded and reduced again, and in third place was the gentle acceleration. I think the test car was a petrol model.
84Dave said:
It's a natural fact within an internal combustion engine..... max Volumetric Efficiency (VE) is very near/at peak Torque rpm. BSFC = combustion/burn EFFICIENCY. -84Dave-
Actually this is not necessarily the case at all. Exhibit A is the Rover 1.8 VVC which makes best BSFC (<250g/kW.h, by the way. I have seen the data) at 3000rpm and 90kPa MAP. The peak torque on the same engine is at 4700rpmThe efficiency of an engine is a balance.
First look at the definition of first law efficiency.
eff = USEFULL work/Heat in
This shows that the efficiency is a balance.
To achieve the maximum combustion efficeincy stoichmetric ratio is required. Without lean burn at low speed you have to close the throttle this gives throttling losses (nothing to do with comp ratio). Lean burn allows you to run a wider open throttle. However you still have stoichimetric combustion just in a local area. It also ensures that complete combustion occurs. It is therefore useful at part load conditions.
At higher speeds pumping losses and friction are dominant. These are not linear with engine speed and there fore at high speeds affect efficiency most.
The most efficient point is the most favourable combination of losses. The peak torque point often occurs near the peak efficiency point on the full load power curve as there is the least amount off losses relative to engine speed. The amount of energy available is proportional to the amount of air sucked into the engine, which is roughly linear to the engine speed. (not quite true as volumetric efficiency is not constant.). So therefore this is where the most torque can be achieved.
There will however be more peak efficiency points at lower speeds however these will be shifted to a different rpm due to the effects of the throttling losses.
Relative to a car (on the flat) there will be an optimum rpm at each speed asuming the same gear and torque required to move the car.
However in a diesel as they do not need a throttle the peak efficinecy point this is just the minimum engine speed required to meet the torque requirement.
Obviously there is more to it when you take into account Turbos and EGR and other systems.
First look at the definition of first law efficiency.
eff = USEFULL work/Heat in
This shows that the efficiency is a balance.
To achieve the maximum combustion efficeincy stoichmetric ratio is required. Without lean burn at low speed you have to close the throttle this gives throttling losses (nothing to do with comp ratio). Lean burn allows you to run a wider open throttle. However you still have stoichimetric combustion just in a local area. It also ensures that complete combustion occurs. It is therefore useful at part load conditions.
At higher speeds pumping losses and friction are dominant. These are not linear with engine speed and there fore at high speeds affect efficiency most.
The most efficient point is the most favourable combination of losses. The peak torque point often occurs near the peak efficiency point on the full load power curve as there is the least amount off losses relative to engine speed. The amount of energy available is proportional to the amount of air sucked into the engine, which is roughly linear to the engine speed. (not quite true as volumetric efficiency is not constant.). So therefore this is where the most torque can be achieved.
There will however be more peak efficiency points at lower speeds however these will be shifted to a different rpm due to the effects of the throttling losses.
Relative to a car (on the flat) there will be an optimum rpm at each speed asuming the same gear and torque required to move the car.
However in a diesel as they do not need a throttle the peak efficinecy point this is just the minimum engine speed required to meet the torque requirement.
Obviously there is more to it when you take into account Turbos and EGR and other systems.
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