Energy per unit CO2 produced, gas vs. diesel

A gallon of diesel contains more combustible material than a gallon of gasoline, which is why you can get more energy per unit from diesel. If the burning process were exactly the same you would end up with the same amount of CO2 per unit of energy.

They are both hydrocarbons; when you burn them you get water and carbon dioxide, plus other byproducts such as nitrous oxide and carbon monoxide. The byproducts come from variations in the combustion process, which varies by engine type.

Tom

 

Hope this helps -

link, select 'energy and emissions' > see GHG > show tailpipe and upstream

Compare Side-by-Side

says Prius is 222 grams/mile*

Jetta Sportwagen TDI is 390 g/mi

natl. avg. for Leaf is 230 g/mi (CO2 only) but 120g/mi in our 94587 zip

* GREET model w/ CO2, NOx and Methane

 

When comparing CO2 per unit of energy they are essentially identical. See Table 3.1 in
http://www.buses.org/files/ComparativeEnergy.pdf

Gasoline is 8,284g CO2/Gallon and 114,000 BTU/Gallon
8,284/114 = 74.4g CO2/1,000 BTU

Diesel is 10,274g CO2/Gallon and 138,000 BTU/Gallon
10,274/138 = 74.4g CO2/1,000 BTU

All else being equal, a diesel produces more work per BTU, so a diesel would produce fewer g CO2 for the same number of horsepower hours .

Which produces the least CO2 per horsepower hour will depend on the design of the engine and how well it was optimized for efficency, but a diesel has an advantage.

 

Close, and maybe you were simplifying...

Diesel has a slightly higher percentage of carbon by weight, but also has a slightly higher amount of energy by weight so they very nearly equal out.

 

Diesel weighs more per unit volume -- about 12% if I remember correctly.

 

I read that as Daniel the first time, and thought, he does sound like kind of a wiry guy.

12% is a good approximation, the number varies because diesel and gasoline densities each vary +/- a couple percent..

 

That's it exactly, and why I said "per unit of energy". I was trying to get the same idea across as your post, but at a very high level.

Tom

 

The biggest difference is a compression ratio 16:1 to 20:1 for a diesel and much less for a gas engine. Prius is a bright spot with 13:1 and their Atkinson Cycle approximation.

Another difference is the lower air pumping loses of a diesel because they don't have throttle plate restricting airflow. Once again though, Prius narrows the gap by using the CVT and electric part of the system to allow the engine to run at wide throttle openings and lower speeds on average than most gasoline engines.

 

You would expect that since the diesel cycle is inherently more efficient. It isn't rocket science; it's straight thermodynamics. When you study engine efficiency, you learn that the theoretical efficiency limit for any engine is directly related to the difference in temperature (delta t) between the hot side of the engine and the cold side. In the case of an ICE, the hot side is the burning fuel during combustion and the cold side is the exhaust.

That sets the theoretical limit. All the other losses subtract from that, but you can't do anything to make an engine more efficient than the theoretical limit.

Diesel engines run hotter. They have higher compression and harder to ignite fuel. This gives them a higher delta t and therefore higher efficiency, everything else being equal (which it never is, but we factor in the details later).

The only way a gas engine can ever get better efficiency than a diesel is if the diesel is burdened with more ancillary losses (the details mentioned above).

Tom

 

Gaso is approx. 85% Carbon and 15% Hydrogen and 0.75 density
Diesel is appox. 87% Carbon and 13% Hydrogen and 0.85 density

Yes same car diesel is better for MPG but more NOx, and if old diesel then particulates etc. Diesel is harder to hybridize. So bottom line I would get the Prius to be near zero emmisions and low GHG.

PS- If you lived in Europe, they set a higher tax on gaso than diesel.
So then you have much less re-fuel cost with diesel. But we do not do that in US.
Diesel costs more even in US but, if we pro-rate by density of fuels, maybe equal.

 

Diesel engines (and here I am generalising) are larger and heavier. They also like to run continuously at a sweet spot. This is fine for some types of hybrids, such as those found in locomotives and ship propulsion systems, but not so good with a family car.

Tom

 

Modern diesels are efficient over a wide speed range.

From Brake Specific Fuel Consumption (BSFC) Maps - EcoModder

The main problem is that in order to meet emissions requirements, the total diesel power plant system has become more complex and expensive. Also also the Atkins like cycle on the Prius has made the Prius engine close enough to diesel efficiency that the additional cost of a diesel isn't justified.

 

Just keep mind GHG emissions of different cars are analyzed by researchers at Argonne Natl laboratory using GREET models. So, it's non-trivial (ok, it's freakin' complicated, I'm sure).

 

First and Second Law by Flanders and Swann.

Flanders and Swann Online - At the Drop of Another Hat - First and Second Law

 

JP government uses following numbers.

		gasoline	diesel
1	energy MJ/liter	34.6	38.2
2	CO2 emission kg/liter	2.322	2.624

Ken@Japan

 

Gasoline at $3.10 costs about $27.2 per MMBTU.

Diesel at $3.70 is about $26.8 per MMBTU.

Of course that doesn't account for the engine thermal efficiency.

FYI, electricity (if you were to use the heat value only) costs $29.3/MMBTU at $0.10 per kWh.

And natural gas (NYMEX today's price) costs $2.54/MMBTU.