Why Diesel Engines Produce Less CO2: A Thermodynamic Approach

[engineer46]

As the title says, why diesel engines produce less CO2 than gasoline engines and how is this related to the more torque and less power in diesel engines?

PLEASE only a fluid dynamics or thermodynamics approach.

Spent hours on searching to find why petrol engines produce more CO2 emissions (or vise versa) but nothing on the web.
I understand the standard explanations on longer stroke, more RPM, higher compression ratio but I am not looking on this kind of stuff.

I am looking for a thermodynamic approach with efficiencies, mass flow rate etc.

Any kind of help will be appreciated.

 

[Mech_Engineer]

why diesel engines produce less CO2 than gasoline engines and how is this related to the more torque and less power in diesel engines?

I guess I should ask the obvious- how do you know diesel engines produce less CO2 than gas engines of similar size and performance?

 

[engineer46]

I guess I should ask the obvious- how do you know diesel engines produce less CO2 than gas engines of similar size and performance?

Today's engines are very different and this can be controvert. I believe that because diesel engines are more efficient, produce less CO2.

I am looking information related on the thermodynamic processes of the two engine types and how diesel (or if you believe petrol, i don't think this is the case) engine produce less CO2 emissions.

According to the power-torque-speed equation, power is function of the amount of fuel going into the engine which is function of the air flow which is function of the speed in size of the engine. But if that was the case, bigger engine=more power or Faster the engine=more power. BUT this is not the case. I believe that the answer relates to some variable which are affected by the way petrol or diesel engine is working. Something to do with the efficiencies (volumetric efficiency and fuel conversion efficiency which both related to power and torque)

 

[Mech_Engineer]

It sounds to me like you're assuming the outcome of a hypothesis and then trying to find supporting data for your assumption...

Diesel engines typically have higher efficiencies than their gasoline counterparts because they have higher compression ratios and turbochargers, both of which increase the thermodynamic efficiency of the engine. That being said, it appears your assumption is wrong according to the DOE. See here:

http://www1.eere.energy.gov/vehiclesandfuels/images/facts/fotw576.gif
http://www1.eere.energy.gov/vehiclesandfuels/facts/2009_fotw576.html

 

[engineer46]

It sounds to me like you're assuming the outcome of a hypothesis and then trying to find supporting data for your assumption...

Diesel engines typically have higher efficiencies than their gasoline counterparts because they have higher compression ratios and turbochargers, both of which increase the thermodynamic efficiency of the engine. That being said, it appears your assumption is wrong according to the DOE. See here:

http://www1.eere.energy.gov/vehiclesandfuels/images/facts/fotw576.gif
http://www1.eere.energy.gov/vehiclesandfuels/facts/2009_fotw576.html

Diesel specific heat capacity is less than the gas capacity. Thus you need more petrol to travel the same distance as in diesel.

Forget about the today's engines, turbochargers, common rail systems, direct injections etc.
Before some years diesels had more miles per gallon than the petrol engines that's the reason all these trucks are diesel engines (more torque-less fuel consumption)

Let's say that I to assume that diesels are more efficient than petrol engines. My questions is somehow a comparison of the typical Diesel engine Vs typical petrol engine of the same displacement.

 

[xxChrisxx]

That being said, it appears your assumption is wrong according to the DOE. See here:
http://www1.eere.energy.gov/vehiclesandfuels/facts/2009_fotw576.html

The only problem with listing a fuels CO2 output vs volume burnt is that it's not really a useful metric.

In terms of motor vehicles you care about CO2 per unit distance traveled. As you travel much further on a gallon of diesel than you do a gallon of petrol, the real world result is that less CO2 is produced over a journey/drive cycle.The trend for diesel and gasoline engine performance and output is actually converging, and both are benefiting from technology crossover. You see this from higher compression petrol engines, and engines running very high boost levels. And much lower compression diesels, such as the skyactiv diesel.
So much so that, aggressively downsized petrol engines are stealing back the bottom end of the diesel market in Europe.

Engineer if you want to explore this from a thermodynamics point of view, just model the ideal cycles on a pv diagram and fiddle with the variables. You can find that in a good thermo book, or a technical automotive engines book.

 

[engineer46]

The only problem with listing a fuels CO2 output vs volume burnt is that it's not really a useful metric.

In terms of motor vehicles you care about CO2 per unit distance traveled. As you travel much further on a gallon of diesel than you do a gallon of petrol, the real world result is that less CO2 is produced over a journey/drive cycle.


The trend for diesel and gasoline engine performance and output is actually converging, and both are benefiting from technology crossover. You see this from higher compression petrol engines, and engines running very high boost levels. And much lower compression diesels, such as the skyactiv diesel.

So much so that, aggressively downsized petrol engines are stealing back the bottom end of the diesel market in Europe.

My questions points ONLY on the typical diesel and petrol engines and a thermodynamic approach behind them.
The only numbers that I am considering is the Fuel conversion efficiency, volumetric efficiency and airflow for both type of engines.

How does these numbers change for diesel and how for petrol. Where those numbers come from and what are the ways to change them (introducing a turbocharger- Is the turbocharger doing the job better in petrol and why)? What is the difference between the diesel cycle and what in otto cycle. How does these numbers can altered to result in a lower emission engine.

These is the stuff I want to hear. Actually my answers are my questions but I am waiting for a person which knows much more than me to give me a complete answer

 

[Mech_Engineer]

In terms of motor vehicles you care about CO2 per unit distance traveled. As you travel much further on a gallon of diesel than you do a gallon of petrol, the real world result is that less CO2 is produced over a journey/drive cycle.

I don't think that quite adds up. If diesel exhaust has about 15% more carbon dioxide than gas, then the engine has to be more than 15% more efficient than a gas counterpart. Without doing any calcs for myself, gas engines are about 25-30 % efficient, and diesels are about 35-40 %. So if anything, its a wash from what I'm seeing...

http://en.m.wikipedia.org/wiki/Engine_efficiency

 

[engineer46]

I will ask my question different:

How CO2 emissions in an engine are produced? What factors affect the emissions?

Is this statement correct: More fuel burned = more CO2 emissions?

 

[etudiant]

I will ask my question different:

How CO2 emissions in an engine are produced? What factors affect the emissions?

Is this statement correct: More fuel burned = more CO2 emissions?

Broadly yes, assuming you are looking at fuel weight, not volume and compare hydrocarbons with only small differences in fuel hydrogen content.
Alcohol based fuels however usually produce less energy and less CO2 per unit of weight.

 

[Mech_Engineer]

It seems to me that since Diesel and Gasoline are both hydrocarbon fuels, the amount of stored energy and CO2 emissions are proportional to the fuel's total carbon content. Diesel happens to contain longer hydrocarbon chains than gasoline (heavier molecular weight, doesn't evaporate in air), so it has more energy per volume but releases more carbon (as CO2 and carbon soot) when burned.

Diesel engines have higher thermodynamic efficiency, which helps make up some for the increased carbon release during combustion, but when painting with a broad brush by generalizing it doesn't seem like either engine will "always" have lower CO2 emissions. Of course I could be wrong, but that's my feeling looking at the rule of thumb numbers.

 

[xxChrisxx]

The problem is the OP appears to have taken his premise from the fact that diesel vehichles of similar peak power output have much lower emissions/ better fuel economy (as what he said holds true 100% of the time). And is then falsely trying to explain this as 100% engine related.

Much of the economy gain is from the way diesel give their output and what this allows you to do further down the powertrain. For example: diesels can run much longer gears due, which means you have downspeeded the engine for a certain road speed.

You also see the same things with new downsized petrols engines. Smaller capacity, boosted (so high low down torque), downspeeded. It's also why they are taking market share back from diesels in the lower power output, B/C segment cars.

 

[HowlerMonkey]

Carbon, as found in CO2 might be less than gas engines but the amount of carbon in other forms such as particulate matter is probably higher than a gas engine.