Why? Just why?

Ardennes1944

Firstly to hold your achtung this is not about me buying a car! :P
Secondly, I wish to point out I know how petrol/diesel engines work etc. My question is though, why can't petrol engines be as economical as diesels? Is it just they're natural make-up that makes it near impossible or is it due to a lack of research/experimenting?

I know it is possible to get diesel-esque mpg from a 1litre lupo but for everyday, ie; 1.6 cars, what's the dealio?
What kind of mpg would diesels 20 years ago have gotten?

TWenty years ago diesels such as the xud and vag 1.6 things got over 40mpg, so did other stuff.

ION08

Deutschlandfürimmer7 wrote: »

Firstly to hold your achtung aufmerksamkeit this is not about me buying a car! :P
Secondly, I wish to point out I know how petrol/diesel engines work etc. My question is though, why can't petrol engines be as economical as diesels? Is it just they're natural make-up that makes it near impossible or is it due to a lack of research/experimenting?

I know it is possible to get diesel-esque mpg from a 1litre lupo but for everyday, ie; 1.6 cars, what's the dealio?
What kind of mpg would diesels 20 years ago have gotten?

I think it's something to do with the fact that diesel automatically combusts at high pressure and doesnt need a spark-plug like petrol does, and the reulting detontaion in the cylinder is a lot stronger so therefore more efficient?

Im sure somebody may have a more informative explanation

Fixed your post by the way

Diesel in automobile land rev lower too as kid torque allows the gearing to be longer

ShadowHearth

Lads, don't you have beds to go and wives to please? I guess only real petrolheads can discuss pricipals of petrol and diesel engines at 2:00 lol

sentient_6

Did some googling out of interest & found this:

http://autotips.plentycar.com/diesel-engine-more-efficient-than-gasoline-engine/


Actually not sure that answers your question as to exactly what cant make a petrol as economical, probably just physical properties of the fuel i suppose. I'm sure they'd have figured it out of they could!

Lumen

Energy densities...

Conventional gasoline: 34.8 MJ/L
Diesel: 38.6 MJ/L

Then there's thermal efficiency, and turbocharging, and the fact that nobody ever gets the manufacturer figures anyway.

Atlantic Dawn

Petrol explodes, diesel burns, that's why.

Ardennes1944

Atlantic Dawn wrote: »

Petrol explodes, diesel burns, that's why.

but the same amount of each enter the chamber for each piston stroke?

W.Shakes-Beer

I think it is down to diesel burning a lot more efficiently compared to petrols.

Petrol cars are only slightly economical (when compared to a diesel) when they are warm, so for the first 10 minutes of a petrol engine running it is rich ie. overfueling.

Diesels don't do that as such.

Thats just my take on it.

But feck it I still love petrols.

sentient_6

Deutschlandfürimmer7 wrote: »

but the same amount of each enter the chamber for each piston stroke?

But diesel has a higher density of hydrocarbons, therefore more energy.

Or put it this way maybe, to produce the same amount of energy less diesel has to be burned, thus more fuel efficient?

W.Shakes-Beer

sentient_6 wrote: »

But diesel has a higher density of hydrocarbons, therefore more energy.

Exactly, the 2 fuels have completely different chemistries, related to the refinement process.

peasant

Lumen wrote: »

Energy densities...

Conventional gasoline: 34.8 MJ/L
Diesel: 38.6 MJ/L

That ..and then there would be the fact that a petrol engine compresses at about 8 or 9 to 1 and a diesel about 20 : 1.
This means that per combustion a diesel engine has about twice the oomph.

(some of which of course is negated by much lower rpm capability)

CarMuppet

Anyone going to mention the flame-front?

Dartz

A petrol engines burns slowly.

A diesel engine detonates, giving that distinctive clatter.

Detonation is a much more violent process, and at a high compression ration you get mucg more whack on the piston slamming it down.

The limiting thing with diesels is that, unlike a petrol engine where the fuel and air mix in the inlet tract before entering the engine, in a diesel they have to mix inside the piston as the cylinder is compressing the mix. As RPM's go up, the fuel just doesn't have enough time to find all the nice juicy oxygen molecules to latch on to.

Ardennes1944

Thanks guys, more or less what I expected. Still, diesel is the devils substance

Esel

Then there's the flame-front . . .

Chuck Stone

I think the detonates versus burns thing is a bit off. Detonation is just burning fast.

Someone corrected me once that nothing explodes in an engine it just burns rapidly.

Deisel has a more energy in it so it takes less to get the same return as petrol.

99nsr125

Deutschlandfürimmer7 wrote: »

Firstly to hold your achtung this is not about me buying a car! :P
Secondly, I wish to point out I know how petrol/diesel engines work etc. My question is though, why can't petrol engines be as economical as diesels? Is it just they're natural make-up that makes it near impossible or is it due to a lack of research/experimenting?

I know it is possible to get diesel-esque mpg from a 1litre lupo but for everyday, ie; 1.6 cars, what's the dealio?
What kind of mpg would diesels 20 years ago have gotten?

Compression ignition versus spark ignition (and carnot cycle etc)
Diesel has a slightly higher energy content but difference in thermal
efficency far outweigh this.

Compression ignition engines are suited to diesel type fuels.

Modern 1.6 petrols have much better fuel efficency than twenty
years ago and an unreal improvement in exhaust emissions.
The improvement in exhaust emmisions hurts fuel economy both petrol and diesel.

Last time I studied the heat cycle a 2-stroke diesel marine engine
was most efficent in the world at ~50% thermal efficency.

The problem is the heat and matierials, current matierials can only
cope with so much and need to be cooled hence lost heat and energy.

A ceramic engine that effectivly doesn't allow heat transfer
ala shuttle protective tiles would give ~95% efficency

Fuel efficeny of diesels twenty years ago are fairly similar to today
however as mentioned above exhaust emmision are much better and available power has doubled.

vincenzolorenzo

Diesel engines run at a much higher compression ratio than a petrol engine (about twice as much) so the thermal efficiency of the engine is higher. There are other things in diesel's favour, but thats the biggie. Compression ratio is limited in petrol engines by the onset of knock aka 'pinking'