Diesel engines -- why?

Tractor trailers are, as far as I know, almost entirely (if not entirely outright) diesel engines. Why is this? Is there something about diesel engines that makes them more powerful?

Some cars, AFAIK, are still made with diesel engines. What’s the deal?

Diesel fuel ignites by itself due to the compression in the cylinder, so there’s no need for spark plugs and stuff.

And that is the sum of my knowledge about diesel.

Diesal fuel is less refined, provides slightly more power, and weighs considerably more than regular gasoline.

The engines use compression to ignite the fuel, thus do not require spark plugs.

Point is, a diesel engine runs at a lower RPM so it uses much less fuel. For tractor trailers, heavy equpiment, ambulances, and boats, it means you can go further on a signal tank of gas. That’s really the big advantage and the main reason its used. Also, by running at a lower RPM the engines tend to last significantly longer. The downside is that they are hard to start, and need to be warmed up longer.

Diesel engines are, by nature, more efficient than gasoline ones. A reciprocating engine runs most efficiently at wide open throttle. A diesel engine is at WOT all the time, and it’s controlled by the amount of fuel going to the engine, not by the amount of air going into the engine.

Deisel engines also make lots and lots and lots of torque, which is more important than horsepower when pulling a load.

The only production cars in the U.S. (cars, not trucks) that are diesel-powered Volkswagons. The Jetta, Golf, and I think Passat can all be equipped by the 1.9 liter direct injection turbo diesel.

The “problem” with diesels for cars (if it can be called that) is that they don’t make much horsepower. A diesel engine will rarely rev over say, 3000 RPMs. And since horsepower is torque X rpm/5252, you’re not gonna have a whole lot of horsepower revving to only 3000.

Hope that helped.

(for more info on VW TDI’s, click here: Click Me! )

Bear in mind that truck engines are much larger and turbo charged compared to cars which usually aren’t. You couldn’t have a 2L, naturally aspirated truck motor so I don’t think they are necessarily more powerful. They are more efficient though as a function of the higher compressions used and of course, diesel is more energy dense than petrol so you get more bang for your buck so to speak.

Diesels have always just struck me as a better design it’s just that they involve a much higher initial investment compared to petrol motors. Now in America, where gasoline is dirt cheap, it’s hard to justify the extra expense of a diesel car but the ruinous petrol prices in the UK or even Germany would definitely have you considering a diesel for your (UK or German) garage. In fact, not only are diesel cars still made, they are becoming more common. Better technology is quickly eliminating many of the old objections like the smell or increased pollution.

In a nutshell: better fuel economy, better durability, better pulling ability to deal with heavy loads. These are particularly advantageous for vehicles that move lots of stuff over lots of miles for lots of years, e.g. tractor-trailers.

Really? Why does the best fuel economy with gas vehicles usually happen around 1800RPM?

The fuel economy for the vehicle is largely related to road speed and air resistance. Any given engine will have its best power/unit of fuel economy at full throttle. The inherent engine economy of full throttle driving in a car is negated by the air resistance found at speeds related to full throttle, which lowers the overall vehicle economy.

Some applications of the engine will allow more full throttle use than others. For example, a stationary power plant engine application can be designed to allow constant full throttle use, whereas a road vehicle application cannot, due to varying vehicle speeds. Freight trucks can’t be at full throttle all the time, but with their large selection of gear ratios they can be run at or close to full throttle more than passengers cars can.

In the interest of the rest of the world (or at least the UK), it’s worth pointing out that we have lots of diesel powered cars.

For many years diesel has been cheaper than petrol and a lot of people chose diesel based on cost. Not me though.

Here’s guide from the AA on running costs which shows the whole-life cost per mile for diesels to be about 80% of the cost for a similarly priced petrol variant.


Thanks for the information, everyone. I think I’m going to dig up some info from How Stuff Works, the operation of a diesel engine seems unclear to me.

Points to elaborate:

Constant wide open throttle; no pumping losses due to overcoming vaccuum at the throttle plate.

Higher compression ratios; each cylinder draws in a little more air and extracts more torque/hp during the expansion downstroke.
Also, HC engines don’t waste as much combustion energy just warming up the cylinder walls.

Lean Burn; combustion takes place in excess oxygen, thus more chemical energy is extracted from the fuel.

Fuel energy density; diesel fuel has more chemical energy density than an equal amount of gasoline.

>> diesel fuel has more chemical energy density than an equal amount of gasoline

There are different kinds of diesel but, in any case, diesel and gasoline products have about the same energy content by mass and that is why specific consumption is generally expressed in grams per unit of energy. Gasoline has somewhat lower specific gravity and lower energy content in the same proportion…

That’s interesting. I thought dieself was a less refined fuel, I’d expect it to have a lower useable energy content. In fact, that’s what I thought fuel refinement was for. Can you help me understand this better?

The “less refined” concept is a mistake. Crude oil in the field is a mixture of C-H-O molecules which start with methane (1 carbon) gas and get heavier as the number of carbosn increases. Gasoline is a mixture of shorter carbon chains then diesel fuel which has somewhat longer chains. The “refinery” is really a process of separating the components by fractional distillation but diesel fuel is no less “refined” than gasoline. Gasoline has shorter chains, is more volatile and more easily inflammable. Diesel is less volatile and less flammable. Because it has longer chains, diesel fuel has higher specific gravity but the energy comes from combustion of the C and the H in the molecules and is therefore proportional (pretty much) to mass.

It is like wood: Per volume (cord) wood energy content varies wildly but per unit of weight the energy content of wood is pretty constant.

I screwed uo. There in no oxigen in hydrocarbons. The generic formula has n carbon atoms and 2*n+2 H atoms. See http://www.gcsechemistry.com/xo7.htm

I screwed uo. There in no oxigen in hydrocarbons. The generic formula has n carbon atoms and 2*n+2 H atoms. See http://www.gcsechemistry.com/xo7.htm

Crude oil is distilled to separate the lighter stuff. After that you get stuff which is too heavy and for which there is not so much demand and part of it can be broken up into lighter molecules by a process called “cracking”. http://www.gcsechemistry.com/xo17.htm

Great information, sailor, thank you very much. Am I just personally ignorant or is it a common misconception that diesel fuel is less refined?

It is possible that some lay people believe diesel fuel is “less refined” but I cannot say how widesread this may be. If anything diesel fuel would be more refined if we are talking about impurities because small impurities in gasoline which would not harm a carburator, if contained in Diesel fuel would harm the injectors.

Also, as I said, there are grades of diesel and fuels are mixtures which are a continuum from methane gas to very heavy oils. Ships diesels use heavier oils than trucks. In my boat, in very cold weather I will mix some gasoline in the diesel fuel to make it more inflammable and therefore start up easier. But you have to be careful with these things and know what you are doing. Diesel fuel is more lubricating than gasoline and not enough lubrication can damage the injectors. Formulating fuels is a complex science and many things have to be taken into account. Add additives and it gets much more complex. For example, some fungus or molds seem to live and thrive on diesel fuel and if you let the fuel sit in the tank for long enough the growth will damage the engine. The answer is to add biocides.

So diesel fuel is as refined as gasoline, if not more, and as carefully formulated, if not more.

The lower flammability is also an advantage and many boats use diesels for that reason. Gasoline is just too dangerous to have on a boat where explosive vapors can accumulate.