On my trusty petrol 2002 1.6L corolla would there be some number of revs per minute that the engine would be most efficient, i.e. best miles per litre? i remember a while back a motoring correspondent said it best to rarely use top gear while dawdling around a city. Would that be connected to getting best mpg ?
it is found by feel, the lightest throttle that will give the required speed without the engine labouring or revving very high, somewhere in the 2000 - 3000 rev area is usual. Generally third gear would be the highest used for rolling along near the thirty limit.
Many cars have a "trip computer" that can be set to show instantaneous mpg.
So a good test would be to drive at a constant speed on a level road at
30mph in different gears to see what you find. My experience with a petrol engine several years ago (I now run a diesel) is that the smallest throttle opening together with the lowest revs gives best economy. So in a 30 limit 4th gave the best figures; but only where there was no need to vary speed or cope with hills.Others here will no doubt report what they find.
0; switch it off whenever possible.
Top gear will give the lowest fuel consumption when you are dawdling at over 1200 rpm and your foot is off the accelerator.
If anyone remembers the old Mobil Economy Runs, the winner always said the same. The highest gear the engine will pull. Odd given that pumping losses are at a maximum with tiny throttle openings. But friction with engine speed must be greater.
Just for interest, I recently changed cars. Both have similar sized engines and BHP and the same auto box. The new one is lighter, but has a much shorter top gear. And despite being some 10 years newer has considerably worse MPG at 70 MPH.
Yeah I remember that once upon a time aeons ago, there was a TV campaign for not to use a lead foot. But this advice was actually proven to be scientifically unsound.
Perhaps it has much worse drag.
Sounds like a simple question, but there isn't a simple answer.
Engine and vehicle manufacturers carry out extensive tests - running an engine against a dynamometer - to produce multi-dimensional maps of an engine's performance - speed vs load vs fuel consumption, etc.
At any given power output, there will be an optimum way of producing this most efficiently. Usually, this will be by using a lower engine speed and higher torque, but is not universally so.
If you have an infinitely variable (within limits!) transmission, it's possible for that to be programmed to choose the best (most fuel efficient) gear ratio for every road speed and load condition. Having said that, the transmission itself won't be as efficient as a manual gearbox - so you'll probable do better by choosing the best fixed ratio in a manual box, even though this may be sub-optimal.
My Hyundai Tucson (manual) tries to do some of the thinking for me by having a dashboard display urging me to change up (usually) or down (occasionally) for greater fuel efficiency.
On 26/03/18 18:00, Graham J wrote: So in a 30 limit 4th gave the best figures; but only where
Agreed.
When I speed up and briefly touch 40mph in my diesel auto on the flat, the box changes to top gear (4th) and I find I can then drop to 30mph 'ish, revs below 2000rpm for economical city driving until scuppered by traffic lights, speed cameras and some hills.
My best mpg however, is typically out on the motorway over a long distance. A mixture of higher revs and road speed, something else is coming into play. (less stopped traffic)
I doubt it. Figures are much the same difference at any speed cruise.
It has a a higher top speed.
If you calculate the gearing difference percentage, the MPG matches it pretty closely.
Yes there will be a certain engine speed, but unless you either put the engine on a test rig or the same engine is sufficiently advanced to be worthy of being the subject of a published paper in an engineering journal then you will not know for sure.
With an instantaneous mpg readout you might gain some insight.
With a gear upshift /downshift indicator on the dash you might get a bigger clue.
Wide open throttle on a petrol vehicle ought to correlate with the lowest consumption as pumping losses are reduced. That is what is done with the likes of the Shell Eco-marathon vehicles. Lots of short period near wide open throttle engine bursts or something with no throttle and direct petrol injection into the cylinders / or a precombustion chamber
Except that WOT tells the engine to produce maximum torque at that engine speed - and will likely use a richer mixture and ignition retard to do so. Over the amount of fuel and ignition advance for best economy.
Yes which is why I went on to say "But road car engine maps and road circumstances rarely accommodate that type of driving"
Diesels have historically had better economy not just because of the higher compression ratio but because they do not have butterfly valves and hence the pumping losses have always been lower.
So you did. Sounds like a bunch of students doing some project with no practical application. ;-)
apart from the older diesels with throttle valves, I never did understand that. and all the modern diesels with butterflys in half the intake, aaargh.
No, it was because they use denser fuels.
Petrol 0.71?0.77 kg/L Diesel 0.832 kg/L That's a ratio of 1.12
Energy in terms of volume - how "we" measure fuel consumption. (Engineers use mass as Kg/Kw.hr and the like) Petrol 34.2 MJ/L Diesel 35.8 MJ/L Not so high ratio 1.04. If you don't use very precise test conditions of an engine engine test cell you won't detect that sort of difference. Installed in a car that's comparable to day to day, run to run variation.
Fuel density is higher due to longer HC chains. It's still alternate single and double bonded ~nH2C. Petrol is around 8H2C or 9H2C and diesel is around 12H2C. Longer chains don't vaporize so well so are useless for SI. Short chains vaporize, short chains like propane C3H8 boil at below freezing, just a bit longer butane C4H10 can be carried in an open bucket at 0°C. Slightly longer chains than Diesel is wax, you will be polishing your furniture and lighting fires with it.
Yes WOT reduces pumping loss and improves efficiency.
Yes higher compression ratio improves efficiency.
The real fundamental gain is that diesels when running efficiently burn all the supplied fuel so don't have pockets of unburnt fuel/air in quench areas that gets pushed out the exhaust port. No fuel leaks past the rings. No fuel drops out of the air flow to wet the port wall. When not running efficiently they don't burn all the fuel, make soot and less power than they would if they ran clean.
That is also why modern gasoline direct injection engine when running in the eco mode can match Diesels. And suffer the same coke build up in the inlet from EGR as Diesels.
Swirl. It's all about swirl.
Shame that the EGR cokes them up and results in perfectly good cars being scraped due to the cost of cleaning the inlet system and repairing the damage the coke does.
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