Ok, this ones for the techie's

Would I be right in stating the a petrol and diesel engine on idle consume the same fuel ratio, it came to me the other day, I thought both say instant consumption 0.2 units, so I would assume the derv gets the extra miles through less revs of the engine when moving, but is just as much a slurper when ticking over?

Edited 15 February, 201016 yr by Supurbia

Diesel has a higher energy content than petrol ... although it's less volatile in terms of combustion. (Which is handy because even petrol contains more explosive power than TNT.)

Diesels are also roughly 10%-15% more efficient at converting thermal energy into kinetic energy.

As for at tick-over. No idea really. You'd think because the engine is more efficient it requires less fuel to maintain the revs to keep it alive, hence at tick-over it'll use less. Sounds logical but then again, if we're using purely logic, bumblebees wouldn't be able to fly.

Edited 15 February, 201016 yr by allclownsareevi

A diesel will always consume less fuel because it works in a slightly different way to a petrol, a petrol engine controller will always try to maintain a fuel to air ratio of 14:1 to provide most efficient fuel usage and will control the power produced by controlling both the amount of fuel and air that enters the engine to give the required power. A diesel controller will control the power of the engine solely by controlling the amount of fuel injected, the air being unthrottled, this is why diesel engines always need a vacuum pump on the engine to provide necessary vacuum for the egr regulator and brake servo. Also a diesel engine will always consume less fuel because it is more efficient, this is due to the higher compression ratio of the engine and the fuel having more energy per litre.

Diesel engines on idle use less fuel than petrol. Where's the extra energy of the petrol going since the car isn't moving? It is going to heat. This is why a petrol engine heats up much quicker.

Everything countryboy said is spot on.

As stated a diesel controls power by controlling the fuel injected. This can include no fuel injected. If you are coming to a stop in gear you use NO FUEL. The roadspeed keeps the engine turning.

As for lower revs, this has a greater effect on the life of the engine. At 100k, a diesel engine will have turned far fewer revs and will have worn far less than a petrol.

Thx for the replies ppl, I'm wondering if anyone has ever done tests though, what you say is true, less revs etc, density of fuel etc, but I wonder what the outcome would be if two engines were benched and tested hot with one gallon, and then from cold with one gallon each to see which spluttered to a halt first, I suspect from whats been said it will be the petrol engine, but it may be a suprise result how quickly the derv engine conks out as well, as an 800 rpm tickover for both negates any on the road movement gains got by the fact the petrol engine would say use 200k revs to the dervs say 130k revs to do the same distance, transmissions play their part as well, the latest Toyota's with 1.8 petrol engines mated to a CVT actually only do one mile per gallon less than the Toyota deisels mated to an autobox do, and it's pretty well the same compairing the CVT to the manual petrol as well, anyway if anyone can find any more info I'd like to read it.

Diesel has a higher energy content than petrol ... although it's less volatile in terms of combustion. (Which is handy because even petrol contains more explosive power than TNT.)

Diesels are also roughly 10%-15% more efficient at converting thermal energy into kinetic energy.

As for at tick-over. No idea really. You'd think because the engine is more efficient it requires less fuel to maintain the revs to keep it alive, hence at tick-over it'll use less. Sounds logical but then again, if we're using purely logic, bumblebees wouldn't be able to fly.

diesel/petrol can be quite close in terms of energy content, theres not much in it....45 vs 44 MJ/kg respectively, but yes the diesel cycles a bit more efficient...they lose less heat and have great expansion ratios (tho also great compression, but they dont cancel out)

diesels dont have throttles, petrols do, so at low-part loads the petrol is losing out due to throttle losses. Also, petrols, thanks mostly to the catalyst, run at 14.7:1 air-fuel ratio......this is kept constant (or should be) no matter what. Diesels however, their air-fuel ratio alters with how hard your pressing the pedal, so the AFR will be rather quite high for an idle diesel.

There's no contest in the specific fuel consumption figures between a petrol engine and a correctly applied diesel.

The major factors in favour of the diesel are the higher compression ratio giving rise to more work being done per stroke, the absence of a throttle, lower heat losses (which includes the fact that no latent heat of vaporisation has to be supplied) and above all else, a non constant air fuel ratio. The fuel has a slightly higher calorific energy value than does petrol.

A good diesel will idle at 50:1 air to fuel as opposed to 15:1 for a petrol. Drive a diesel gently at part load and the gains are substantial.

Millions of lorries, trains and boats can't be wrong. (Diesels are a bit heavy to fly...).

rotodiesel.

diesel/petrol can be quite close in terms of energy content, theres not much in it....45 vs 44 MJ/kg respectively, but yes the diesel cycles a bit more efficient...they lose less heat and have great expansion ratios (tho also great compression, but they dont cancel out)

diesels dont have throttles, petrols do, so at low-part loads the petrol is losing out due to throttle losses. Also, petrols, thanks mostly to the catalyst, run at 14.7:1 air-fuel ratio......this is kept constant (or should be) no matter what. Diesels however, their air-fuel ratio alters with how hard your pressing the pedal, so the AFR will be rather quite high for an idle diesel.

You're working from KG (mass) not litres (volume). Diesel is around 38.6 MJ/litre whilst petrol is 32 MJ/litre. Around 20% more energy content per volume. Would like to say I knew this already but I had to look up those values.

Engine SFC is measured using fuel weight.

rotodiesel.

Engine SFC is measured using fuel weight.

rotodiesel.

Really? Figured it'd be by volume. Not as if you buy fuel by the kg. I stand corrected.