Why do diesels have lower CO2 emissions than petrols?

miniyazz

miniyazz

miniyazz

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I always rather assumed that the reason diesels get better fuel economy is due to higher energy per litre as a result of larger average molecule size/higher molar density.
But if this were the case, they should be producing more CO2 per litre of fuel which should then roughly balance out.

What gives, oil gurus?
 
You are right in saying 1 litre of diesel is denser and has more joules of energy in it than 1 litre of petrol.
Diesel engines are more efficient than petrols by quite a higher margin. I'd be making it up if I said what the percentage efficiency between the two is.

The funny thing is, CO2 is not the main concern coming out the back of diesels, it is clouds of particulate matter.
 
And NOx, which is why diesel will be expensive in the coming years as they will be taxed to hell. they are far more polluting that petrol.
 
Glaucus said:
And NOx, which is why diesel will be expensive in the coming years as they will be taxed to hell. they are far more polluting that petrol.
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My MX5 has no NOx - It says so on the MOT certificate :p.


I work in a sector who is introducing off-road machinery, with new emissions as of Jan this year.

(Note: Timeline not accurate).
Six years ago mechanical ignition met the emissions. Smokey as hell but run on any old rubbish diesel.
Three years ago the emissions were met by retarding the ignition. Engines get a little more picky on fuel quality.
Three months ago a DPF was good enough, Engines even more picky on fuel.
Now there are three backboxes - DPF, some urea doser and a catalyst. These will cost a fortune to maintain.

Africa, India etc. still use the old mechanical non-dpf machinery. If in a large room image one smoker is in the corner, it eventually ruins everyone's air quality - The world is exactly like this.
 
The world isn't like this at all. Seeing as mist pollution is local.
Which is why pollution in city centres is far higher than outside. As it gets caught between the building. so that statement is utter rubbish. If what you said is true, then pollution would be very uniform, which it isn't in the slightest. As from the London thread a 2014 diesel has to meet the same NOx emissions as a 2006 petrol.

EU is taking the UK to court as we have not met air pollution targets in many cities. Not even close to meeting them. so expect far more schemes to come in, to try and meet them.
 
They don't. Diesel engines produce approximately 13% more co2 than petrol engines and if a 2 litre turbo diesel engine could produce the same power as a 2 litre petrol engine it would no longer hold any benefit in fuel consumption
 
jej_knowles said:
They don't. Diesel engines produce approximately 13% more co2 than petrol engines and if a 2 litre turbo diesel engine could produce the same power as a 2 litre petrol engine it would no longer hold any benefit in fuel consumption
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No they don't CO2 is directly proportional to fuel usage diesels use less fuel and therefore produce less CO2. Hence while a lot of diesels are in a lower tax band than there petrol counterpart stupid really as diesels are much more polluting in terms of NOx and particulates.
 
jej_knowles said:
They don't. Diesel engines produce approximately 13% more co2 than petrol engines and if a 2 litre turbo diesel engine could produce the same power as a 2 litre petrol engine it would no longer hold any benefit in fuel consumption
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Rubbish, and what's more, it's torque that is important, not power (torqueRPM) and turbodiesels will trounce their petrol equivalents here.
 
snapdragon69 said:
Rubbish, and what's more, it's torque that is important, not power (torqueRPM) and turbodiesels will trounce their petrol equivalents here.
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:rolleyes: So if it were possible, you reckon a 1000 lfbt 10bhp engine would make a quicker car than one making 1000bhp and 10 lbft? I am pretty certain that modern turbodiesels are the main cause of people utterly failing to grasp the concept of torque and power.

The reason the diesel produce smaller amounts of CO2 then equivalent petrol engines is down to the diesels greater thermal efficiency, i.e. they convert more of the fuels chemical energy into mechanical energy, primarily due to the very high compression ratio a diesel can use and partly due to the lack of throttling losses.
 
snapdragon69 said:
Rubbish, and what's more, it's torque that is important, not power (torqueRPM) and turbodiesels will trounce their petrol equivalents here.
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2005 Fiesta Zetec-S, 1.6 Petrol

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2005 Fiesta Zetec-S, 1.6 Diesel

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2005 Fiesta ST, 2.0 Petrol

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The ST is 3 seconds faster on the 0-60 sprint than the 1.6 TDCI, dispute having less torque.

So yes, obviously power is not important :rolleyes:
 
And diesels generally produce more torque than equivalent petrol engines, as the moving masses are greater- diesel engines have to be made of steel as they run too hot to be made of aluminium. The pistons, cranks etc therefore have more momentum, incresing their torque output.

(*Apparently!)
Blimey, I learnt something from Top Gear...
 
dougal1331 said:
And diesels generally produce more torque than equivalent petrol engines, as the moving masses are greater- diesel engines have to be made of steel as they run too hot to be made of aluminium. The pistons, cranks etc therefore have more momentum, incresing their torque output.

(*Apparently!)
Blimey, I learnt something from Top Gear...
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The main bonus (aside from the ability to tow stuff easily) of more torque, is fewer gear changes are needed. If you were doing 20mph, in third in a petrol car, and you put your foot down to get to 60mph, you'd probably need to drop to second gear, then go back up. A diesel would just pull when you put your foot down.

EDIT Whoops, wrong button. Shoulda been edit, not quote, but hey-ho...
 
When combusted petrol produces 2392g of CO2 and diesel produces 2640g of CO2. As you are combusting less diesel, you are producing less CO2.

Broadly speaking, a turbo diesel will out-perform it's normally aspirated petrol equivalent measured in peak power terms, as the turbo diesel will produce more power lower in the rev range and you must use the rev range when performing any acceleration in anything short of a CVT. Lots of gears helps mitigate.
 
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Slower but more thorough "burn" means less revs required = lower emission or something like that.

Otherwise heavy trucks would use petrol engines !
 
Glaucus said:
The world isn't like this at all. Seeing as mist pollution is local.
Which is why pollution in city centres is far higher than outside. As it gets caught between the building. so that statement is utter rubbish. If what you said is true, then pollution would be very uniform, which it isn't in the slightest. As from the London thread a 2014 diesel has to meet the same NOx emissions as a 2006 petrol.

EU is taking the UK to court as we have not met air pollution targets in many cities. Not even close to meeting them. so expect far more schemes to come in, to try and meet them.
Click to expand...



Yes of course there is a concentred smog in cities, but CO2 is not photochemical smog (dubbed the LA smog in California - In a valley so doesn't easily get blown away). The world is one big 'room' - It does not matter where the CO2/NOx is emitted from in the long run. Remember the industrial revolution, which has caused the damage is only just over 100 years old, which in the lifetime of the planet is merely a blink of an eye.
If we made all cars in London electric, it would certainly help the air qualtity in London, but all you are doing is moving the 'tailpipe' (coal power plant) away from the city, so whilst the city pollution improves, the overall picture pretty much stays the same.

I should get a Landrover V8 with a 4 litre engine that pumps all the exhaust fumes into a gas tank, then empty it in the countryside, because it makes the city clean and all :p :).

I worked as an environmental manufacturing researcher for a year (very boring. Do not recommend it).
The environment is like politics. Everyone has an opinion. Most are wrong.

Another spanner in the works is at what point does a new 'eco' car break even environmentally against an old one.
Say you have a 2 litre 1998 Mondeo that works just fine, you scrap it, dig up material for a new car, process it, assemble it....then suddenly you have an economical car - If you focus on the 'use' phase on the lifecycle definitely, but is it really any better?
 
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jej_knowles said:
They don't. Diesel engines produce approximately 13% more co2 than petrol engines and if a 2 litre turbo diesel engine could produce the same power as a 2 litre petrol engine it would no longer hold any benefit in fuel consumption
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Parkers indicates that for a BMW 320d/i, as the first example (there is 7 BHP difference (less than 5%) with the diesel at 160bhp and petrol 167bhp), 0-60 @ 7.7 vs 7.4s, so pretty closely matched - diesel has 109g CO2 vs petrol @ 124, almost 15% less. However I grant, a litre of diesel should emit about 13% more CO2 than a litre of petrol..


I appreciate that NOx and SOx etc are the real emissions issues but I considered CO2 to be a better indicator of actual energy released :)

Greater thermal efficiency resolves all my queries :p
 
Sorry my Mistake I meant to say,
If a 2 litre turbo diesel could produce the same power as a 2 litre turbo petrol it would no longer hold any benefit in fuel consumption.
Or both naturally aspirated.
 
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dougal1331 said:
And diesels generally produce more torque than equivalent petrol engines, as the moving masses are greater- diesel engines have to be made of steel as they run too hot to be made of aluminium. The pistons, cranks etc therefore have more momentum, incresing their torque output.

(*Apparently!)
Blimey, I learnt something from Top Gear...
Click to expand...

The civic 2.2d engine is an aluminum block.

Also even my S2000 will pull cleanly from 20mph in 3rd.
 
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