Viewing: Why is diesel so efficient

***waynne*** · 19-05-2011, 07:01 PM

I had this question emailed to me this morning from one of our readers so I thought i'd post it up as it will make for an interesting discussion.

"Why does diesel allow a car to travel further per mpg than any other fuel on a molecular level."

***HDi fun*** · 19-05-2011, 07:18 PM

One - diesel engines do not have throttles so there is theoretically no energy expended drawing air into the cylinders. It's for this reason that they also respond very well to turbocharging - the other way to look at it is that without turbocharging they're not great fun to drive either!! Also, a diesel engine driver flat out at high rpm is not likely to be much more economical, if at all, that a petrol driven the same way. NASP diesels are plain thirsty when driven hard.

Two - the brake mean effective cylinder pressure is very high due to the high compression ratio and lack of a throttle.

Three - and this is where some compromises can be unveiled. Diesel has a higher calorifi value than petrol by VOLUME. ie. 1 litre of diesel contains more energy than 1 litre of petrol.

Read on - IF the fuel companies were to meter fuel by mass rather than by volume then diesel would lose out somewhat.

It is significantly denser than petrol. So 60kgs of diesel would give less litres of fuel than 60kgs of petrol.

**Yugguy** · 20-05-2011, 07:58 AM

I also think part of the reason is relatively low displacement and relatively low rpm. Big engines and high revs take more air/fuel mixture in

***T9 man*** · 20-05-2011, 08:43 AM

I have always had this thing about diesel engines, silly I know, as they are a far cry from the diesel engines of my dads era - were thick clouds of smoke out the exhaust and noisey cabin intrusion was the norm.

Oh, Congrats on reaching your first 1000 TC posts Yugguy

**turbonutter69** · 20-05-2011, 09:18 AM

Quote:

Originally Posted by T9 man

I have always had this thing about diesel engines, silly I know, as they are a far cry from the diesel engines of my dads era - were thick clouds of smoke out the exhaust and noisey cabin intrusion was the norm.

Oh, Congrats on reaching your first 1000 TC posts Yugguy

I have that same thing.

Just can't get it out my head that diesels are tractors.

***HDi fun*** · 20-05-2011, 10:44 AM

That's just tribalism.

***T9 man*** · 20-05-2011, 10:49 AM

Quote:

Originally Posted by turbonutter69

I have that same thing.

Just can't get it out my head that diesels are tractors.

Run for it TN, i'll try & hold them off for as long as I can!

**turbonutter69** · 20-05-2011, 10:53 AM

Quote:

Originally Posted by HDi fun

That's just tribalism.

You know what HDi?
It's been along time since we had this debate...
Game on...

Petrol all the way.....Down with tractors.

***HDi fun*** · 20-05-2011, 11:11 AM

Don't have time at the moment. Just moved house.

Ringside seats available Friday 27th

**turbonutter69** · 20-05-2011, 11:18 AM

Hows the move going buddy?

***HDi fun*** · 20-05-2011, 12:38 PM

Quote:

Originally Posted by turbonutter69

Hows the move going buddy?

Cardboard boxes and bubble wrap everywhere.

Any fetishists should apply soon

**jarrus** · 22-05-2011, 11:16 PM

Diesels are better, the end....

Diesel engine cars romped in the BTCC and in LeMans, fact!

**jarrus** · 22-05-2011, 11:16 PM

Seriously though, slower burn rate is one reason diesels are so much more economical, also they are far less wasteful than petrols, because you always gave a full volume of air in the cylinder and diesel engines are accelerated by adding more fuel, so adding less fuel in the same volume of air equals less waste.

**turbonutter69** · 23-05-2011, 12:02 AM

Quote:

Originally Posted by jarrus

Diesels are better, the end....

Diesel engine cars romped in the BTCC and in LeMans, fact!

When diesels romped BTCC was when the petrol cars was all NA. Not a comparison.

**jarrus** · 23-05-2011, 12:45 AM

I reckon they would still give the turbo petrols a run though.

**turbonutter69** · 23-05-2011, 12:54 AM

The petrol turbo's was dominating this years BTCC last time I watched it.

**jarrus** · 23-05-2011, 01:07 AM

O yeah, they are, plato keeps complaining about it. There aren't any diesels competing currently though...

**turbonutter69** · 23-05-2011, 01:12 AM

There you go then.

**jarrus** · 28-05-2011, 10:03 AM

I think cost comes into it, VAG proved there point then left it there.... They mainly use diesels in there motorsport ventures...

Grizbear · 20-08-2011, 12:19 PM

Two words, " power density", retail designs rev low because of high compression.... Diesel fuel packs more of a proverbial punch than an equal mass of gasoline, alcohol, CNG ... It burns slower so therefore it revs slower, but it has more extractable power than gasoline hence the high torque numbers(ie: cylinder pressure ) as compared to a conventional spark ignited engine, chamionship Le Mans racing has gone the way of compression ignition for this reason.... More power from less fuel , Btw new Audi lemans car(VAG) is a 6 cyl TDI...

**jarrus** · 20-08-2011, 10:58 PM

Erm, no...it's a V12 mate,

Grizbear · 20-08-2011, 11:27 PM

Naw Naw homie it's a 3.7L v6, if u don't believe me... Look here
http://www.autoweek.com/article/20101211/ALMS/312119999
7th paragraph

**Marc07** · 21-08-2011, 09:09 AM

Quote:

Originally Posted by Grizbear

Naw Naw homie it's a 3.7L v6, if u don't believe me... Look here
http://www.autoweek.com/article/20101211/ALMS/312119999
7th paragraph

If I could get the Diesel technology from that car installed in mine, then that would be probably the first time I would want a diesel my self

I know that this days diesels are much much better and much more powerful and you get even more after remapping......but I just love the petrol engines, petrol smell

sound of a petrol engine and everything about it.
Yes, diesel engines last maybe longer, but are more expensive to fix in some aspects, they are more economical....but I do not really care, Petrol all the way baby

**turbonutter69** · 21-08-2011, 09:19 AM

Quote:

Originally Posted by Marc07

If I could get the Diesel technology from that car installed in mine, then that would be probably the first time I would want a diesel my self

I know that this days diesels are much much better and much more powerful and you get even more after remapping......but I just love the petrol engines, petrol smell

sound of a petrol engine and everything about it.
Yes, diesel engines last maybe longer, but are more expensive to fix in some aspects, they are more economical....but I do not really care, Petrol all the way baby

Diesels don't always last longer than the petrol variant. It's how you maintain your car that counts not the fuel it uses.
Also I don't agree that diesels are more economical because they just aren't.

**Marc07** · 21-08-2011, 09:30 AM

Quote:

Originally Posted by turbonutter69

Diesels don't always last longer than the petrol variant. It's how you maintain your car that counts not the fuel it uses.
Also I don't agree that diesels are more economical because they just aren't.

Agree on the first one, sort of, but Diesels are more efficient, more MPG than petrol equivalent, I'm not saying that by a lot (same engines), but they are .......or not !?!?