I have always wondered why heavy vehicles such as large trucks and buses almost always use diesel engines, whereas in light cars there is a choice between diesel and gasoline engines.

Engine efficiency might explain the preference towards diesel engines, but then again isn't this a valid consideration also in light cars? You should get better efficiency with a diesel engine, and therefore, all cars should use diesel engines.

So, why do large vehicles almost always use diesel engines, but light cars have a choice between gasoline and diesel?

Could modern technology such as hybrid technology with Atkinson cycle bring gasoline engines to large vehicles as well? I have read that the Atkinson cycle engine in 2016 Toyota Prius has 40% thermal efficiency. This is very diesel-like in my opinion and if the technology is scaled up, could help bring gasoline technology to large vehicles.

  • Manufacturers also make gasoline engines because there is a demand for them from people who don't want a diesel engined car. – HandyHowie Dec 27 '15 at 21:40
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    Aren't there two stroke and four stroke diesel engines? What kind of diesel engine are you asking about here? – Ppoggio Dec 27 '15 at 22:21
  • In Switzerland, gasoline buses and trucks are common. – sergiol Apr 1 '16 at 23:43

Torque is the name of the game. High torque is needed to move heavy loads. If comparing a gasoline engine to a comparable diesel engine the diesel will always have higher torque. The higher torque comes from the need for a higher compressing ratio needed for compression ignition. To achieve the higher compression ratio a longer stroke is required. The longer stroke comes from a greater crankshaft offset. This offset gives greater torque.

Another aspect is that diesels can make tremendous torque at very low RPM. Very simply put more fuel equals more torque when everything else is kept the same. A diesel does not have throttle plates and draws in the maximum amount of air on every stroke. In a diesel the amount of fuel added is what controls the power. The throttle controls how much fuel is added. This means that a diesel always runs lean. At idle the engine uses hardly any fuel. This lean mixture allows for the addition of large quantities of fuel even at low RPM. A gasoline engine on the other hand always has to keep the fuel mixture at optimal stoichiometric. This need to keep the mixture correct means that to get more fuel the engine needs to rev to higher RPMs. This means that a gasoline engine makes it's torque at much higher RPM than a diesel. This high end torque characteristic makes a gasoline engine hard to drive necessitating constantly keeping the RPM high.

The only real draw back to this torque production is a limited RPM. This is compensated by a gear box with lots and lots of gears.

If a gasoline engine was used it would have to be much larger. The much larger engine would make for greater fuel consumption.

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    Don't forget that just as a diesel engine can be geared up, a gasoline engine can be geared down to increase torque. I like the explanation about the engine internals but it doesn't really explain the entire system design. – altendky Dec 28 '15 at 0:35
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    @altendky I think his answer is sufficient. He didn't go into an elementary explanation, but if you have basic knowledge of physics; You should be able to understand exactly what he's trying to convey. I'm not insinuating you don't, but If someone wants a VERY in depth answer about the physics and mechanical function/ engineering of a diesel engine it wouldn't be a bad question if we have any scientists or engineers on here! We are a mixed bunch haha – cloudnyn3 Dec 29 '15 at 15:24
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    I have to agree with @altendky. Torque isn't the answer. It's the fact that diesel engines are inherently more efficient – Zaid Dec 29 '15 at 18:04
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    @cloudnyn3 I can get umpteen Nm of torque from a motorcycle engine and a big gear ratio, yet they do not use them in heavy vehicles. So, torque alone is not the answer. – altendky Dec 29 '15 at 22:57
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    @Zaid An argument can be made that a diesel is the most efficient at making the needed amount of torque. – vini_i Dec 30 '15 at 0:08

A major, but often overlooked, reason for the dominance of gasoline engines in passenger vehicles is the need for diesel engines in heavy vehicles. A given quantity of crude oil, depending on its composition, will yield a given quantity of diesel, a given quantity of gasoline, a given quantity of candle wax, and specific given quantities of other petroleum products. Therefore, if we fix the amount of any one of these products that we need as constant, then we've also fixed all the other products' quantities as constant as well.

As vini_i explained in another answer, diesels produce more torque as a byproduct of the engineering decisions in creating a higher compression ratio engine. The heavy vehicles get the diesel-powered torque-producing motors. Let us fix the amount of diesel fuel needed to move a given nation's goods via diesel. Now, we've got a fixed quantity of gasoline that was produced as a byproduct of producing that diesel fuel. It makes much more sense to put gasoline engines in the passenger vehicles which can burn this leftover gasoline, than to put diesel engines in the passenger vehicles which will then compete for the fixed quantity of diesel being produced.

Thus, neither engine type would be economically well suited for use in all vehicles, regardless of technical considerations. Whenever fuel for one engine type is created, fuel for the other engine type is created as a byproduct and someone will come along with an engine that can burn that byproduct.

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    I think you have addressed a very key point here that has been missed. Nice contribution. – DucatiKiller Dec 28 '15 at 17:42
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    When will candle wax engines become commonplace? – Octopus Dec 29 '15 at 22:55
  • @Octopus: In fact the root of the word kerosene (i.e. jet fuel) is the Greek word for wax! – dotancohen Dec 30 '15 at 0:12
  • This doesn't explain why diesel passenger cars are so popular in Europe. I haven't heard of deals where Europe sells unused gasoline to the US. – Dmitry Grigoryev Feb 9 '16 at 14:02
  • In Brazil, Diesel is even forbidden for light passenger vehicles. – sergiol Apr 1 '16 at 23:38

To OP’s main question: “Why do heavy vehicles almost always use diesel engines?” Answer: Cost and dependability. Diesel engines are significantly more expensive, but have lifetimes many times greater than gasoline engines. For a commercial vehicle that is on the road all day every day, it adds up to big savings because of the better fuel efficiency and less downtime for repairs.

To the OP’s additional question about light vehicles using diesel: In the United States, diesel has a very negative consumer image, that it's dirty, loud, slow, etc. Car companies claim that even if diesel powered consumer vehicles are superior to gasoline, consumers in the US won't buy them. It's not worth debating the validity of that claim here, but it is worth noting that in other places such as Europe, a large percentage of consumer vehicles are diesel. So the answer may be more cultural than scientific (whether I mean US consumer culture or Big Three car company culture is open for interpretation).

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    I think the recent diesel emissions scandal from VW and other European auto manufacturers has furthered the perception of diesel as bad. – DucatiKiller Dec 28 '15 at 16:02

Given two engines of similar weight, both operated at their respective optimum efficiency (i.e. maximum mechanical work done per unit of chemical enthalpy in the burnt fuel), you will end up with similar fuel consumption for either engine type. But a Diesel engine will generally offer slightly more power out of this, by giving more torque; that's how it's more efficient.

However, such optimum efficiency is always reached at pretty low RPM. Now, piston engines actually offer most power at high RPM, albeit at the cost of reduced efficiency. I.e., by shifting down and revving up, you get substantially more power (and need much more fuel). Now, because Otto engines can be revved higher than Diesel engines (and also tend to respond much faster), they are more suitable for this kind of “overclocking”, and therefore rather more attractive for sports cars. For trucks, this isn't economic though.

If you will, an Otto engine is a compromise between a Diesel engine (heavy; good efficiency at low RPM; little extra power at high RPM) and a gas turbine (very light; terrible efficiency at low RPM; lots of power at high RPM).

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    and the greater torque comes because of this: Gasoline contains mainly alkanes (paraffins), alkenes (olefins), and aromatics. Diesel fuel consists mainly of paraffins, aromatics and naphthenes. The hydrocarbons of gasoline contain typically 4-12 carbon atoms with boiling range between 30 and 210 °C, whereas diesel fuel contains hydrocarbons with approximately 12–20 carbon atoms and the boiling range is between 170 and 360 °C. Gasoline and diesel fuel contain approximately 86 wt-% of carbon and 14 wt-% of hydrogen but the hydrogen to carbon ratio changes somewhat depending on composition. – shabby Dec 28 '15 at 7:30

Another consideration, in the United Kingdom at least, is that you can buy "red diesel" (diesel fuel, dyed red) for use in agricultural use, stationary generators etcetera with far less tax. Currently diesel here is around £1.10 per litre (local garage, rode past earlier) whereas the last time I bought red diesel it was around £0.60 per litre.

I do not know if this sort of thing occurs in other countries but given the similarity in diesel/petrol (gasoline) performance this significant reduction in fuel costs means that diesel is the fuel of choice for tractors and agricultural machinery or any engine that doesn't drive the wheels of road-going vehicle, such as refrigeration systems and generators etcetera. If a similar thing occurs in other countries this is no doubt one reason that diesel engines dominate these industries. You would not be able to make a petrol engine that would use sufficiently less fuel over its lifetime to recoup such a fuel cost differential.

I don't know but I would venture to suggest that such a situation arose years ago when petrol engines simply couldn't produce the required torque or reliability at the required conditions so diesel was the preferred option. Legislation on fuel tax for different uses will not keep up with modern rates of development by private companies.

Granted, the red diesel argument doesn't cut it with road going vehicles, but when you consider the load pulled in a 40' trailer is comparable to that of a tractor and trailer in a field it is easier for engine manufacturers to make two similar engines tailored to the market. I don't know anywhere near enough about who owns who when it comes to diesel engine manufacturers over the past fifty years but it is at least some food for doubt.

Finally, if everyone used diesel, the police would have to check every vehicle for red diesel in the fuel tank, rather than the small percentage that could feasibly use it today. This would costs governments and oil companies many millions in lost revenue, which would be unthinkable of course.

  • This does occur in other countries which apply taxes for road fuels (i.e. quite a lot of them.) The Republic of Ireland uses green dye instead of red, which means that those living on the Northern Ireland border can mix Irish and UK agricultural diesels to give a brownish colour that looks natural to the naked eye. In this way they hope their use of agricultural diesel on the road will go undetected. – Level River St Dec 29 '15 at 23:51

Some good and some almost good answers here.

Diesel (or any piston) engines - contrary to that claimed above - do not necessarily need a long stroke for high compression; but they often have under square bores and/or strokes that are not short (by typical gasoline engine standards).

Diesel engines - contrary to that claimed above - do have throttle plates; these regulate the incoming air flow - which in turn regulates torque, power, and revs . . etc.

High compression can be achieved with and without altering the stroke, and this can be done by changing the con-rod length (not the same as stroke) so it places the piston further up the cylinder block and possibly protruding into the combustion chamber, and/or by altering the piston geometry.

Greater stroke can and does often lead to a greater instantaneous and composite torque moment at the crankshaft (think leverage with a big wrench; as this [plus more friction] is exactly what you give the BMEP and/or force exerted on the piston produced by each combustion product when it is connected to the crankshaft with a long stroke/"lever"); but - as mentioned - it also increases friction.

Diesel engines typically can run an extremely high compression (much higher than typical gasoline engines) due to the fuel they use that - amongst other things - doesn't compression-ignite at compression ratios that would in a typical gasoline engine.

As the responses and notes from above say; the name of the game is always Torque.

And that - plus the ability to produce high torque figures both reliably and (for the work done) economically - is why Diesel engines are made.

HorsePower (a force that is related to objects [a truck] that move in a straight line and/or linear plane) is simply a product of torque (a force associated with objects [a crankshaft] that spin and/or rotate); the amount of torque produced in a given time and/or revs.

Since there really are on a few things the powertrain-engineer can do to increase the output of any (diesel/gasoline) piston engine (aside from, increasing fuel burnt and/or swept capacity, increasing static engine capacity, increasing compression, reducing friction/reciprocal weight . .etc), please note that increasing compression is directly related to both increasing efficiency and output.

Unlike, say, increasing the static engine capacity; as a 454 cubic inch chev (<7 litre) is not necessarily more efficient than, say, a modern 3 litre V6 - despite the 454 probably chev probably being more powerful - provided the 3 litre V6 was not turbocharged.

Even then, turbocharge the 454 and you will have prodigious torque and power beyond 1500HP and possibly approaching 2000HP provided all tuning/fuelling is done right.

So Diesel engines are designed to produce significant torque via the above-mentioned design approaches and very high compression/combustion.

The length of the stroke within a diesel engine has more to do with maximum torque generation (from the combustion product) and/or design - than pure compression; but - as stated above - it can also assist the compression.

The combustion process is a complex one, and this is one place where efficiency, economy, and torque/power can be maximized.

This is why we see modern day gasoline cars - especially the European ones - all coming out with direct injection; like most diesels have had for years.

As this way the combustion process can be better controlled for all conditions and driving "modes".

Diesel engines - unlike most typical gasoline engines, especially those of a decade ago - almost always ensure that they fire precisely at top dead centre (TDC) due to the fact that they rely on compression-ignition.

Many typical gasoline engines - some of even today - are without the precision to fire every single combustion cycle right at TDC due to the complexity of an engine and how fast things inside move; and when this fails to happen efficiency and torque soon drop.

The faster an engine spins the harder it is to ensure that every single combustion cycle fires right at TDC; this is one reason why non-diesel engines these days all have individual coil packs (for each plug) and some form of electronic/computer controlled ignition.

Comparably, Diesel engines don't need electronic ignition systems at all, and they don't have high crankshaft speeds either (an ocean liner diesel will rarely do more than 250 - 300 rpm, if that).

Diesel engines are also basically built to generate significant torque from very low vehicle/engine speeds, and they also use a fuel that was designed (back when the only gasoline fuel available was leaded) to facilitate a very high compression ratio.

It is a fallacy that Diesel engines are way more efficient than typical modern day gasoline engines.

Usually - a decade or so ago - it was the diesel engine's ability to provide significant torque from low vehicle/engine speeds, combined with its ability to support high compression ratios, and also the fact that diesel's were turbocharged; that often provided the perceived efficiency and other advantages over the typical gasoline engine.

These days - especially with non-leaded gasoline products that support high compression - typical gasoline engines are not only turbocharged, direct injected and running high compression ratios - but they're also capable of both, greater crankshaft rotational speed bandwidths than diesels and also producing big torque numbers at low crankshaft rotational speeds, too.

That hits the same target as several of the unique selling propositions that diesels previously offered.

Still, the Diesel motor will enjoy popularity for a while longer as Diesel fuel is slightly cheaper than typical gasoline.

Plus, diesel engines; (a) are robust, (b) are relatively simple, (c) they usually run at low speeds [and are therefore torque products and "reasonably" economical/cost effective], (d) they don't require sophisticated valve-train and/or ignition system considerations, and, (e) when they're designed to operate in 2-stroke mode they can be implemented to yield greater torque outputs with sometimes equal or less complexity, particularly with respect to valve-train considerations.

That said, I think, the combination of the slow demise of the fossil fuel industry, most first world countries' carbon/pollution policies, and also the rise of hybrid/stand-alone electric motors within passenger vehicles, will probably - unless it significantly evolves - kill the diesel engine off within the next 10 years.

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    I'm skeptical of the claim that diesel engines will get killed off in the next 10 years. I think you will get better traction on your response if you break it up into sections a bit. tl;dr – DucatiKiller Dec 28 '15 at 16:18
  • I'll recant my "throttle plate" suggestion here ... it does make sense to have a throttle plate on a diesel in order to get the EGR to work better. The idea here is to create more vacuum, not to limit the amount of air going into the system (which is the primary reason on a gasoline engine). I'm surprised they just don't introduce more overlap into the cam profile. This allows reversion to occur which pretty much does away for the need of an EGR valve in the first place. I still stand by the "rant" suggestion, though. – Pᴀᴜʟsᴛᴇʀ2 Dec 29 '15 at 15:34
  • Here's a question and place to post regarding diesel throttle plates. mechanics.stackexchange.com/questions/23956/… – DucatiKiller Dec 29 '15 at 17:07

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