Engine efficiency
Discussion
I have wondered for a while now why two cars with different sized engines weighing the same, travelling at the same speed and for the same distance would have different fuel consumption.
Surely both vehicles require the same amount of energy.
A 150 bhp engine would for example be using 100 bhp for the given speed. That is 66% of available power. A 400 bhp engine in theory needs the same 100 bhp for the same given speed which is only 25% of the available power but will apparently have worse fuel consumption. Can anybody explain to me why this is
Many thanks
Surely both vehicles require the same amount of energy.
A 150 bhp engine would for example be using 100 bhp for the given speed. That is 66% of available power. A 400 bhp engine in theory needs the same 100 bhp for the same given speed which is only 25% of the available power but will apparently have worse fuel consumption. Can anybody explain to me why this is
Many thanks
Speary8 said:
I have wondered for a while now why two cars with different sized engines weighing the same, travelling at the same speed and for the same distance would have different fuel consumption.
Surely both vehicles require the same amount of energy.
A 150 bhp engine would for example be using 100 bhp for the given speed. That is 66% of available power. A 400 bhp engine in theory needs the same 100 bhp for the same given speed which is only 25% of the available power but will apparently have worse fuel consumption. Can anybody explain to me why this is
Many thanks
Pumping losses, I believe. A bigger engine at part throttle is being strangled (via smaller throttle opening, thus using more energy and therefore fuel to simply breathe) more than a smaller engine at a wider throttle opening.Surely both vehicles require the same amount of energy.
A 150 bhp engine would for example be using 100 bhp for the given speed. That is 66% of available power. A 400 bhp engine in theory needs the same 100 bhp for the same given speed which is only 25% of the available power but will apparently have worse fuel consumption. Can anybody explain to me why this is
Many thanks
Larger cylinder or more cylinders? If the former then they'll be an optimal cylinder size to produce that amount of power. Anything larger or smaller than that will be less efficient (thermodynamically - more heat loss due to surface area, compression ratio, pumping losses due to throttle in a SI engine etc.) This is for a given bore/stroke ratio. An 1.5l F1 engine would be much less efficient that a 4.0 V6 road engine!
Edited by fido on Thursday 19th May 22:23
Smaller is generally better due to pumping losses, but a larger engine also more internal friction as more valves, larger piston surfaces, more oil and coolant being pumped around.
However downsizing only works in certain scenario's:
If it's a tiny 1.0 Ecoboost running at near peak power to produce the 100hp required it could even be less fuel efficient than say a 1.5 or 2.0 Ecoboost. The turbocharged engine would be running rich to keep the combustion temperatures lower.
The reality is for cruising the maximum legal speed on the motorway you only need about 40-50hp depending on the aerodynamics of the car, width of the tires and drivetrain losses such as a 4*4 system.
The same engine could be frugal in a Fiesta but awful in a fully loaded hybrid Transit with an empty battery at motorway speeds or when trying to pull a caravan up the Alps...
However downsizing only works in certain scenario's:
If it's a tiny 1.0 Ecoboost running at near peak power to produce the 100hp required it could even be less fuel efficient than say a 1.5 or 2.0 Ecoboost. The turbocharged engine would be running rich to keep the combustion temperatures lower.
The reality is for cruising the maximum legal speed on the motorway you only need about 40-50hp depending on the aerodynamics of the car, width of the tires and drivetrain losses such as a 4*4 system.
The same engine could be frugal in a Fiesta but awful in a fully loaded hybrid Transit with an empty battery at motorway speeds or when trying to pull a caravan up the Alps...
Aerodynamics are a big thing.
You get similar efficiency difference with the same engine in different design cars.
Few years ago wifey had a Citroen C2 VTS 1.6HDi, cracking little car that went well albeit too hard riding.
Frugal up to about 60mph, beyond that and it started to use a lot of fuel, worse at 70 or above than a much heavier C5 with the same engine, circa 15mpg worse at 80mph, aerodynamics playing a big part here as top gearing not a great deal different though the enine mapping might be completely different.
Remapping for increased torque to map out the annoying lack of oomph below 1800 rpm improved things a little.
You get similar efficiency difference with the same engine in different design cars.
Few years ago wifey had a Citroen C2 VTS 1.6HDi, cracking little car that went well albeit too hard riding.
Frugal up to about 60mph, beyond that and it started to use a lot of fuel, worse at 70 or above than a much heavier C5 with the same engine, circa 15mpg worse at 80mph, aerodynamics playing a big part here as top gearing not a great deal different though the enine mapping might be completely different.
Remapping for increased torque to map out the annoying lack of oomph below 1800 rpm improved things a little.
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