Why are diesel engines considered better than gasoline engines?

[Vivee=)]

I hear diesel engines are better than gasoline engines. Why is that?

 

[FredGarvin]

Better at what?

 

[mgb_phys]

I hear diesel engines are better than gasoline engines. Why is that?

They are more efficent in extracting more of the chemical energy of the fuel - ie more miles per gallon. They can also use lower grade fuels which need less refining and so should be cheaper (they aren't because of taxes).
Diesel engines don't need ignition systems so can be simpler but must run at higher pressures and so need to be stronger, this makes them last longer but are more expensive to build.
They emit more particulate pollution but less NOx - opinions differ as to wether they are overal better for the enviroment

 

[brewnog]

Diesel: more efficient, higher torque for lower speeds, no inherent reliance on electricity. Heavier, and have the need for more expensive ignition systems. Very receptive to turbocharging.

 

[NateTG]

Diesel: more efficient, higher torque for lower speeds, no inherent reliance on electricity. Heavier, and have the need for more expensive ignition systems. Very receptive to turbocharging.

Diesel engines do not have more expensive ignition systems, but they do have significantly more expensive fuel injection systems. Moreover, diesel engines are not intrinsically heavier than otto engines, but, rather, tend to run at higher compression (which is also why they work so well with turbochargers).

Diesel engines are more efficient because they can run at higher compression, which works out to better thermodynamic characteristics. Otto cycle engines would have premature ignition (a.k.a. knocking) at those pressures.

Historically diesel engines have been heavier in order to accommodate the higher compression, but with improved materials and injection technology, automotive diesels are quite competitive with otto cycle engines in terms of specific power these days. Marine diesel engines can have pretty spectacular performance characteristics http://people.bath.ac.uk/ccsshb/12cyl/

 

[brewnog]

Diesel engines do not have more expensive ignition systems, but they do have significantly more expensive fuel injection systems.

Ah, thanks. Slip of the tongue.

Moreover, diesel engines are not intrinsically heavier than otto engines, but, rather, tend to run at higher compression (which is also why they work so well with turbochargers).

The higher compression ratios (and higher cylinder pressures) necessitate the core components to be stronger, and (generally) as a result heavier than a spark ignition engine of similar output. In addition, the traditional expectation of a Diesel engine is for much longer life, though this comes largely from the historical commercial background of Diesel engines.

 

[Ivan Seeking]

Diesels have greater fuel flexibility. The first diesel ran on peanut oil.

Diesel fuel has a higher energy density than gasoline

 

[perkhouse]

Diesels have greater fuel flexibility.

True!

The first diesel ran on peanut oil.

Not true! The first engine Dr. Diesel designed ran on coal dust, but he couldn't make the injection system work reliably, so he tried another alternative fuel: vegetable oil.

I have 3 diesel vehicles: '01 Jetta TDI, '82 Mercedes, and '86 Isuzu. They all 3 run on high percentages of vegetable oil. The VW has been a vegetarian for almost 20k miles now.

 

[mgb_phys]

The diesel engine was invented and first built by an English guy called Stuart.
Fortunately his name didn't catch on or we would be talking about a Turbo-Stuart-Intercooler or Stuart-Dykes.

 

[perkhouse]

I thought Stuart was an Aussie.

 

[Ivan Seeking]

The diesel engine was invented and first built by an English guy called Stuart.
Fortunately his name didn't catch on or we would be talking about a Turbo-Stuart-Intercooler or Stuart-Dykes.

On February 27, 1892, Diesel filed for a patent at the Imperial Patent Office in Germany. Within a year, he was granted Patent No. 67207 for a "Working Method and Design for Combustion Engines . . .a new efficient, thermal engine." With contracts from Frederick Krupp and other machine manufacturers, Diesel began experimenting and building working models of his engine. In 1893, the first model ran under its own power with 26% efficiency, remarkably more than double the efficiency of the steam engines of his day. Finally, in February of 1897, he ran the "first diesel engine suitable for practical use, which operated at an unbelievable efficiency of 75%.

Diesel demonstrated his engine at the Exhibition Fair in Paris, France in 1898. This engine stood as an example of Diesel's vision because it was fueled by peanut oil - the "original" biodiesel[continued]

http://www.ybiofuels.org/bio_fuels/history_diesel.html

So it is a matter of how you define "first".

Is it even possible to get 75% efficiency out of a diesel? He must have been running a fantastically high compression ratio if true. I think the best that I've seen in large generator applications is just over 60%. This was based on fuel consumption rate, fuel energy density, and a generator driving a known electrical load. If we take the generator to be 90% efficient including the engine coupling, then the engine would be operating at about 67% efficiency.

Ironically, peanut oil isn't biodiesel in the literal sense. Biodiesel is a fuel that can be made from just about any plant oil.

 

[Ivan Seeking]

I have 3 diesel vehicles: '01 Jetta TDI, '82 Mercedes, and '86 Isuzu. They all 3 run on high percentages of vegetable oil. The VW has been a vegetarian for almost 20k miles now.

Fantastic! Do you mix vegey oil with diesel fuel directly?

What is your normal range of ambient temperatures? Do you have trouble starting the engines in cold weather?

 

[mgb_phys]

Is it even possible to get 75% ef... The best modern marine diesels do about 58%.

 

[mgb_phys]

I thought Stuart was an Aussie.

Nay lad e wer a yorkshire man. Although he lived in Oz and built engines there.
There isn't a monument or anything because the English don't like inventors ( Turing invents the computer and gets a mini-roundabout named after him) but some company in Halifax named a model of parking bollard after him!

 

[brewnog]

Whoah, 60% is rather impressive! Any chance of a reference please?!

 

[mgb_phys]

This is the most efficent and most powerful internal combustion engine http://people.bath.ac.uk/ccsshb/12cyl/
There is a GE engine for power generation that is slightly more than 60% efficent but it is a gas turbine + diesel combined heat and power unit - since you use the hot exhaust for heating you get more efficency than if you just counted the shaft horse power.

 

[Ivan Seeking]

Whoah, 60% is rather impressive! Any chance of a reference please?!

I will dig up some links later. IIRC I was looking at large, two-stage diesel turbine generating stations [1 MW+ range]. When I got to the very large systems, there was a big jump from the typical 40% efficiency that I see for smaller engines.

 

[Ivan Seeking]

I did some looking and didn't spot anything yet. I was thinking it was Caterpiller, but they have the 500 hp multi-fuel compatible, diesel turbine [an interesting find]. Their 3125 KVA gen units were only checking out at 42% total efficiency.

I try again more later. I should have a link somewhere.

 

[perkhouse]

Fantastic! Do you mix vegey oil with diesel fuel directly?
What is your normal range of ambient temperatures? Do you have trouble starting the engines in cold weather?

I have a processing "plant" in my garage where the VO is washed, filtered, dewatered, filtered, blended, filtered, and pumped (while being filtered again) through a gas-station-type nozzle into the vehicles.

I live in central North Carolina where the summer temps are around 70/90 and the winter temps are around 25/40. I blend with a concentration of D2/VO of around 20/80 in the summer and 60/40 in the winter. I do use block heaters almost every day of the year which costs probably around 20 cents per day. And I'm adding electrical fuel pumps and 12V injection line heaters right now which should allow me to increase my percentage of VO regardless of the season.

Nay lad e wer a yorkshire man. Although he lived in Oz and built engines there.

Although it's clear he lived in England during his mid-years, I'm pretty sure he lived in Oz when he was young, but I'm not sure where he was born. We know he lived in Oz again when he was older. (Returning "home"?)

 

[mgb_phys]

According to the anu he was born in halifax (uk) and moved to Oz in his 30s to start his own company http://www.adb.online.anu.edu.au/biogs/A120145b.htm

The 60% efficiency is for a diesel fas turbine not an internal combustion engine.
http://www.power-technology.com/features/feature1084/

There are diesel engines being designed for tanks which use peltier stacks to generate electricity from the waste exhaust heat - if you include the electricity generated they are > 60% efficent.

 

[brewnog]

I did some looking and didn't spot anything yet. I was thinking it was Caterpiller, but they have the 500 hp multi-fuel compatible, diesel turbine [an interesting find]. Their 3125 KVA gen units were only checking out at 42% total efficiency.

Thanks Ivan. I can definitively say that it's not Caterpillar!

 

[Ivan Seeking]

Okay, I think I might have this sorted out. First, based on a number of selections up to 100 MW in size, I'm showing no better than 44% efficiency for diesel generators. This would suggest that the engines are doing as well as 48% efficiency. Gas turbine generators are cited as having efficiencies as high as 60%.

It seems that my reference was to the latest in oil fired, steam turbine systems, such as here. I had never seen this one before, but it is representive of what I had in my notes.

An electrical generating plant of high efficiency utilizes a conventional steam plant powered by a fossil fuel such as coal, gas or oil, in internal integration with a high temperature solid-oxide fuel-cell. In one embodiment, the spent fuel and the wast heat from the fuel-cell of electrochemical action is made directly available to the combustion furnace of the steam plant for thermodynamic extraction. The system can achieve efficiencies up to 65% compared to ordinary steam plants which have an efficiency of about 35%.

http://www.freepatentsonline.com/4644751.html

I may chime back in on this later as some information still doesn't jive. I could swear that I saw as much as 60% efficiencies on very large diesel turbine generating stations, but at this point it seems likely to have been for diesel fired steam with heat recovery systems.

 

[minorwork]

Just a reminder that a gallon of diesel has more btus than gasoline and the engine does not throttle the intake air, thus inherently lessening the pumping losses that a gasoline engine has at light loads.