Diesel or Petrol ?
Discussion
Well it's a pub conversation I've had a couple of times which seems to boil down to economics - the dieselites think they are saving money - and don't seem to have any concern about the environment.
I wondered if someone would know
the answer to a question that gets asked - can you build a car with petrol engine that has the same performance (speed, acceleration, fueld econmony) as the same car but with a diesel engine ?
Diesel cars seem to invariably have turbos now to boost their performance - but could you for example detune a turboed petrol engine to give the same limited performance characteristics in trade for better fuel economy ? Or does it boil down to whether diesel or petrol ignites more efficiently ?
I wondered if someone would know
the answer to a question that gets asked - can you build a car with petrol engine that has the same performance (speed, acceleration, fueld econmony) as the same car but with a diesel engine ? Diesel cars seem to invariably have turbos now to boost their performance - but could you for example detune a turboed petrol engine to give the same limited performance characteristics in trade for better fuel economy ? Or does it boil down to whether diesel or petrol ignites more efficiently ?
a turbo makes an engine more efficient - as a result performance may increase but primarily it boosts effiency... ie more bang per buck.
most diesel equivalents are bigger cc than the same model petrol. I have no problem with oil burners for either - short trips (no plugs no cat no need to run for ten miles to reach effiency) or for high mileage cruisers .... esp a bmw 330 ! but thats just me
most diesel equivalents are bigger cc than the same model petrol. I have no problem with oil burners for either - short trips (no plugs no cat no need to run for ten miles to reach effiency) or for high mileage cruisers .... esp a bmw 330 ! but thats just me
sorry but its all to do with fuel energy content and engine efficiency. Litre for litre diesel has a higher energy content than petrol which means when you buy a litre of diesel then you're effectivly buying more energy. If that makes any sense to you. An extreme example of energy content of fuel per litre would be to look at a litre of cryogenic Hydrogen. It only weighs about 70 grams (as opposed to about 800grams for petrol) and as a result doesn't give you much energy when you burn it.
Also its to do with the way it is burnt in an engine. The most efficient way to burn fuel in an cylinder engine is to compress it first. The more you compress it the better the efficiency (up to a point). Petrol engines are limited in their compression ratios by the anti knock properties in the fuel. If you try and compress petrol too much then it will pre ignite and damage your engine.
Diesels on the other hand have higher compression ratios so are therefore more efficient.
So unfortunately looks like diesel engines are better for economy.
Pity they are the devils own creation. I hate them with a passion. You'll notice that americans hardly bother with them as car engines because they have cheap petrol. This is deffinently the way forward.
ban all diesel cars I say. If you want to save the environment run your car on LPG FFS.
anyway hope this wasn't too technical or long winded for you. Just be glad I didn't start banging on about lowest entropy points etc etc.....
Andy
Also its to do with the way it is burnt in an engine. The most efficient way to burn fuel in an cylinder engine is to compress it first. The more you compress it the better the efficiency (up to a point). Petrol engines are limited in their compression ratios by the anti knock properties in the fuel. If you try and compress petrol too much then it will pre ignite and damage your engine.
Diesels on the other hand have higher compression ratios so are therefore more efficient.
So unfortunately looks like diesel engines are better for economy.
Pity they are the devils own creation. I hate them with a passion. You'll notice that americans hardly bother with them as car engines because they have cheap petrol. This is deffinently the way forward.
ban all diesel cars I say. If you want to save the environment run your car on LPG FFS.
anyway hope this wasn't too technical or long winded for you. Just be glad I didn't start banging on about lowest entropy points etc etc.....
Andy
I am afraid that diesel and petrol are two very different methods of generating power. Ok, so they use a combustion engine that is similar in principle, but one is forced ignition while the other is spark induced.
I think that there are various characteristics that need to be considered with petrol and diesel. Petrol engines are smoother, lighter and in general more powerful. The use of clever electronics can produce good fuel economy, but diesel will always be better. However, a diesel engine will be noisier (the explosions are less controlled than in a petrol engine), heavier (the compression rates in the blocks are so much higher you need to run cast iron engines (though please do note that the next generation diesel engines are starting to get some very weird alloy parts to reduce weight) and finally less powerful.
However, the use of common rail technology and turbos you can get towards the same specific output levels of a similar petrol engine. The characteristics are different but the end result is generally the same. For example the guide is currently like the following:
"a 2.0 petrol engine will generate more power but less torque than an equivalent 2.0turbo diesel engine".
The performance will be approximately the same with the only real difference being at the top speed. But the differences in the speed increments will be even when you work out across the full range....
Can petrol be as economical as diesel? Probably, but the demands of performance with petrol mean that this isnt really the push at the moment. What I would say is that there are more technical (electronic and physical) changes that can be made to a diesel engine to boost power and economy than a petrol one...we are only now getting 16v 4 cylinder diesel engines now!
Only my 2p worth though...
Cheers,
Paul
I think that there are various characteristics that need to be considered with petrol and diesel. Petrol engines are smoother, lighter and in general more powerful. The use of clever electronics can produce good fuel economy, but diesel will always be better. However, a diesel engine will be noisier (the explosions are less controlled than in a petrol engine), heavier (the compression rates in the blocks are so much higher you need to run cast iron engines (though please do note that the next generation diesel engines are starting to get some very weird alloy parts to reduce weight) and finally less powerful.
However, the use of common rail technology and turbos you can get towards the same specific output levels of a similar petrol engine. The characteristics are different but the end result is generally the same. For example the guide is currently like the following:
"a 2.0 petrol engine will generate more power but less torque than an equivalent 2.0turbo diesel engine".
The performance will be approximately the same with the only real difference being at the top speed. But the differences in the speed increments will be even when you work out across the full range....
Can petrol be as economical as diesel? Probably, but the demands of performance with petrol mean that this isnt really the push at the moment. What I would say is that there are more technical (electronic and physical) changes that can be made to a diesel engine to boost power and economy than a petrol one...we are only now getting 16v 4 cylinder diesel engines now!
Only my 2p worth though...
Cheers,
Paul
andytk said:
ban all diesel cars I say. If you want to save the environment run your car on LPG FFS.
Andy
I agree, but I use LPG because its cheaper. It is after all Luquid Petroleum gas, which is a finite resource as we all know.
Damage to the enviroment is reduced using LPG via the end product - the car, but it still requires extraction (electricity or diesel runs the machinery, and supports the workforce) and not forgetting transport (diesel trucks), in fact it requires a greater resource to deliver it to the stations than diesel or petrol as the supplying stations are spread over a wider area.
LPG or even CNG (compressed natural gas) are not the future for us or our children, and I dont know what is.
pbrettle said:Can petrol be as economical as diesel?
Of course it can, if you're talking cost per mile rather than mpg, as the pitch is completely queered by the tax that's charged at the pump.
There's a reason you see so many diesel cars in Europe - diesel is 1/2 the price at the pump, which more than makes up for the differential in servicing costs & purchase price (where they exist).
Gargamel said: a turbo makes an engine more efficient - as a result performance may increase but primarily it boosts effiency... ie more bang per buck.
The point of a turbo is to compress the air so that more fuel can be burnt in each power stroke of the cylinder, increasing performance but using more fuel. I don't think this necessarily equates to being more efficient, as you're providing more power to the vehicle but at the expense of burning more fuel.
{edited to add:} ah well, I might be confusing efficiency with economy here. Mind you, I doubt there's many 996TT drivers out there getting the same kind of mpg as the AX diesel grandads...
>> Edited by pdv6 on Thursday 7th November 15:05
So diesel has in theory more energy than petrol and can be run at a higher compression than a petrol engine. So it boils down to whether you can make a petrol engine run more efficiently than a diesel (sounds unlikely). Can running a petrol engine in "lean burn" mode compensate (do diesels have to run at 14:1 air / fuel) ?
I just don't want to have to buy a T440TDR
in 10 years time because everyone has given up on petrol.
I just don't want to have to buy a T440TDR
in 10 years time because everyone has given up on petrol.
Ok, let's start by getting the terminology right.
Otto cycle ("petrol") vs Diesel cycle.
as has already been pointed out, diesels have a higher compression ratio, increasing efficiency.
Diesels also have no throttle, reducing pumping losses, (as most of the time we're not at full throttle...)
However... direct injection gasoline engines will also eventually be u throttled to all intents and purposes, reducing losses...
And variable compression ratio will to some extent further improve gasoline efficiency.
what was the question again?
Otto cycle ("petrol") vs Diesel cycle.
as has already been pointed out, diesels have a higher compression ratio, increasing efficiency.
Diesels also have no throttle, reducing pumping losses, (as most of the time we're not at full throttle...)
However... direct injection gasoline engines will also eventually be u throttled to all intents and purposes, reducing losses...
And variable compression ratio will to some extent further improve gasoline efficiency.
what was the question again?
andyvdg said: Can running a petrol engine in "lean burn" mode compensate (do diesels have to run at 14:1 air / fuel) ?
yes - GDI is getting towards this. The problems are getting stable combustion and reducing NOX emissions.
diesels do not run a fixed AFR, they are inherently lean burn.
Well I'm off to burn some petrol whilst I can.Gassing Station | General Gassing [Archive] | Top of Page | What's New | My Stuff