Why do diesels have lower CO2 emissions than petrols?

I thought the reason for diesels generally being steel engine blocks was to do with the much higher compression required so there was a requirement for the engines being stronger.
 
They're iron not steel! The cylinder pressure and resulting torque is the reason on most engines. And cost, all the other gubbins already drive enough cost into the difference above petrol variants.
 
Internal Combustion Engines generate Torque, Turbo Diesels Make Lots.

Engines don't make Horse power only horses do that :) we convert the Torque measurement bringing time into equation so we can compare it to a horse for some reason :D

Torque is what it's all about, HP exists so tuning firms can without anything more than raising RPM limits increase the HP number without making any more power...you can't mess with real power measurement like that.
 
Hp exist to overcome drag as you actually need a car to do work to sustain road speed.... Workrate = power. If you pop back to a 'tuning company' or indeed any OEM; check if they have a clue in a cupboard, maybe you can take it home with you.
 
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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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Torque is the most important you are right, but what you failed to recognise is that it is NOT peak torque, it is the torque curve and over what RPM range it can maintain that torque.

Diesels have a narrow torque band (ok for it's RPM range) and run out of steam at very low RPM, (big drop off at much lower RPM than a decent petrol) a product of the compression ignition. A petrol engine can vary it's ignition timing to maximise the potential energy of the fuel so can maintain a lower peak torque over more of the RPM range, which is why we look at power (work rate) as well as Torque.

Torque is increased by having a greater stroke but that is counter productive to RPM as shorter stroke engines rev higher.

Oh and the flywheel/crank mass is usually greater on a diesel which is good at low rpm but parasitic in terms of power output.

Now then you have to take into account "effective" torque, ie that which makes it to the road, and that is where the gearbox comes in. The gearbox is a torque multiplier, using the gear ratios to increase torque at lower speeds while utilising the engines RPM to maintain effective torque.

Try both cars on a rolling road in 2nd gear, torque output will appear very different, hence why they always use a high gear giving closer to a 1:1 ratio.

A motorcycle engine is a great example of this, all motorcycles I am aware of have a primary drive that can (example) double the torque from the crankshaft to the gearbox by halving the RPM. Hence even with very low torque at the crank they can output big HP and amazing performance.
 
OpenToSuggestions said:
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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This is what most of the self important tree-hugging hippy and eco-warrior types seem to forget when they preach about their brand "low emission" Prius.

I drive a car with a relatively big petrol engine, but it's 11 years old and I only do about 6k/year. I could scrap it for something new and more economical/less polluting, but how long would it really take to see the benefits, both financially and in terms of pollution?
 
Janesy B said:
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:
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Just FYI, you've referenced a 2 litre petrol against a 1.6 litre diesel....retard much :p
 
Haggisman said:
This is what most of the self important tree-hugging hippy and eco-warrior types seem to forget when they preach about their brand "low emission" Prius.

I drive a car with a relatively big petrol engine, but it's 11 years old and I only do about 6k/year. I could scrap it for something new and more economical/less polluting, but how long would it really take to see the benefits, both financially and in terms of pollution?
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Do i go down the internet cliche or Clarkson regurgitation aisle to find one those 'preachers', yet to actually find one in real world.
 
Jonnycoupe said:
Do i go down the internet cliche or Clarkson regurgitation aisle to find one those 'preachers', yet to actually find one in real world.
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I guess you must be right. After all, you know everyone, so since you've never met one, everyone who has must be lying.

Or would could look at the legislation being looked at to restrict older cars being allowed to access central London as an example of encourage people to scrap perfectly good cars in order to reduce emissions.

Or the number of people who get rid of, again, perfectly good cars, to buy small engine "eco" cars because they get to save £100 a year on tax due to lower emissions.

You do realise cliches and stereotypes don't just come out of nowhere, right?
 
Shame they don't levitate above the pot holes that they are not contributing to the repair of :D

If anyone else marketed a product as zero emission could you imagine the consequences.
 
Haggisman said:
I guess you must be right. After all, you know everyone, so since you've never met one, everyone who has must be lying.

Or would could look at the legislation being looked at to restrict older cars being allowed to access central London as an example of encourage people to scrap perfectly good cars in order to reduce emissions.

You do realise cliches and stereotypes don't just come out of nowhere, right?
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2005 is old enough isnt it? I think the word you are looking for is pollution, not emissions.

Central London? Who cares, the place is hardly somewhere you would want to drive a car.

You do realise cliches are over used phrases. Key operative part of that is the overused.
 
Jakus said:
Shame they don't levitate above the pot holes that they are not contributing to the repair of :D

If anyone else marketed a product as zero emission could you imagine the consequences.
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Nissan then, which they do; unlike Toyota with the Prius, which they don't.
 
The concern for me with diesels is the nano diesel particulates. They still don't know what long term these have on health, but I bet in the future they will be found to be very dangerous.
 
Problem is the petrol ones are actually smaller hence you dont really see them! I think they are of a different composition but as smaller tend to be find deep in the aleovi.

EU6 was at risk of direct injection petrols driving in the requirement for GPFs (Gasoline) to manage particulate matter. Im not sure where they stand at the minute but it does guarantee compliance, can allow the removal of a silencer and no where near as complex to regenerate as a DPF that requires high temp drive cycle.
 
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