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engine revs for best petrol economy

Except the best Shell eco cars have specially built small engines that can be as small as 40cc and are matched to the small light weight super aerodynamic single seat "cars" and 15mph average speed over 10 miles of the competition. The other engine of choice is a small motorbike engine typically under 200cc.

You would be very upset if you bought a car and then found it couldn't exceed 30 mph or pull off on the flat with 4 people aboard. Anything bigger than a sub 1L (Smartcar etc) may well be expected to tow a trailer.

To do as you suggest with a typical small car such as the 1L Ford Ecoboost Fiesta would in top gear result in speeds in the range of

70-100-70-100mph. That goes to show how over powered and excessively big the 1L "Eco"boost engine is for 30-40mph town driving that a "shopping trolley" does.

The best economy is achieved in the highest gear it will "pull". By "pull" I mean it still responds to throttle. If the throttle is opened just a touch more and it just makes noise and fails to respond then the gear is too high for the road speed, every little throttle movement will just waste fuel.

Best BFSC for SI engines is at peak torque (peak efficiency, a wide range of engine speeds these days with VVTi etc.) and about 3/4 throttle. For a huge number of cars this can't be achieved on the UK road as it would be in excess of the NSL and M-way speed limits. Peak torque on my car is 4000rpm and that's like 90mph in top.

Get a vacuum gauge. Learn how to use it. Or you can buy a bluetooth ODBII device for a few £ of E-bay (close to £20 for Ford/Mazda as they need a special Forscan switched version) and download Torque lite to your Android phone and then show vacuum and load gauges. (iPhone no free lite version it's gonna cost you)

Reply to
Peter Hill
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Yes diesel is more energy dense but only by about 12% 38MJ/L vs 34MJ/L, yet the differential in mpg between petrol vehicles and diesel vehicles has always been significantly more than that. Heavier internals too in a diesel engine.

Reply to
The Other Mike

Can't speak for any 'older diesel' with a throttle valve. What were they fitted to?

Modern diesels fo that for emissions purposes and not primarily for fuel mixture control.

The vast majority of diesel engines for the best part of the last century and a quarter meter just fuel and do not throttle the air.

Reply to
The Other Mike

bedford TK has a throttle valve, so yes, it is older, but not ridiculously so.

Reply to
MrCheerful

Diesel engine used in automobiles is typically much better than 35.8, whatever some random person has put into Wikipedia.

e.g.

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Reply to
Nick Finnigan

Yes so 10-12% of the better mileage is to do with having more calories per litre I never did understand why the higher tax on diesel was not maintained.

The remainder is to do with better volumetric efficiency at part power because there is no throttling of the air sucked in. This means the smaller amount of fuel at part power is still injected into a full compressed charge of air which is heated up and all then expands after the piston descends. With the spark ignition engine only a fraction of the air:fuel mixture is imbibed and thus a smaller mass of heated air drives the piston down from a lower initial compression pressure.

As thermodynamic conversion efficiency is directly related to the pressure from which the gases expand and the throttled spark ignition engine is working with lower pressures the compression ignition engine remains more economical at part power.

I gather that some multi cylinder spark ignition engines are getting around this by using hit and miss, i.e. shutting the valves sequentially on cylinders and thus keeping the working cylinders working harder at part power.

It is also part of the reason hybrids can be more efficient as their smaller engines are working harder most of the time.

I have recently found that at a steady 60mph my fiesta eco diesel is about the same economy as a similarly 8 year old Honda civic hybrid at just over 60mpg.

AJH

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