Gasoline vs Diesel engine

One cannot be non-fascinated by the performance of a car engine. This engine works. If it doesn’t start running at the moment the key is turned, we instantly get shocked, paralyzed and aroused by disbelief.

Sure, the reason it works so well is simple - years of perfecting.

Yet, there are so many basic things about gasoline and diesel engines that I learned only recently. I listed some of them here, hoping that you will find something interesting also.


All engines are engineered around fuels, so first we have to discuss fuels. Both, the gasoline (petrol) and the diesel are hydrocarbons. The “only” difference being the (average) length of the hydrocarbon chain - diesel being longer. Thus, they have different properties:

    - gasoline is lighter (less dense) and less viscose than diesel
    - gasoline vaporizes more readily than diesel at room temperature. This makes gasoline a bit more dangerous because it easily creates enough-concentrated vapors that are easy to ignite into fire.
    - diesel is somewhat better in lubricating things than gasoline

In the engine, fuel is well mixed with air and burned. Therefore we have to say something about air-fuel mixture properties: the diesel-air mixture will ignite at lower temperature than the gasoline-air mixture. The gasoline-air mixture can simply withstand higher temperature before self-ignition. This is a very important thing to remember. It is also a bit confusing.

In everyday experience, it seems that the gasoline will much easier engage into flame then the diesel. But remember that this is only because the gasoline vaporizes more readily at room temperature. It means that, at room temperature, there is much higher concentration of gasoline vapors than diesel vapors and therefore the gasoline will ignite more easily. However, when we consider same concentrations of gasoline and diesel vapors, then the diesel will ignite easier (lower temperature is needed to activate combustion). Inside the engine, the temperature is high enough for both, the gasoline and the diesel to vaporize completely and create rich-enough (flammable) concentrations of vapor.

Now we know something about fuels. Lets talk about engines.

Engine basics

Mister Rudolph Diesel was aware of the gasoline engine (Otto cycle) problems and wanted to improve it. The gasoline engine inherently has problems with efficiency and/or fuel.

In order to improve the efficiency one must increase the compression ratio of an internal-combustion engine (see the bonus section at bottom of this article). However, in the gasoline engine there is a limit – the gasoline-air mixture will self ignite once the compression gets too high (because every compression drives temperature increase). So, either you can have a low-efficient, low-compression engine that uses a cheap fuel, or you can have a high-efficient, high-compression engine that uses expensive, high-refined fuel that wont self-ignite even at high compression levels (a 120 octane gasoline?).

In diesel engine this problem is solved. The diesel engine can use much higher compression levels than the gasoline engine reaching higher efficiency. In addition, the diesel engine can use fuel that is not nearly as refined as the high-octane gasoline fuel (thus cheaper).

To make this possible, Rudolph changed the Otto cycle and created the diesel cycle. The difference is that during compression phase, no fuel is present in the cylinder and thus no self-ignition can happen. The fuel is only injected at the moment the ignition is wanted – when injected into the hot pressurized air the diesel fuel self-ignites immediately (the diesel-air mixture, as we said already, is happy to ignite even at relatively low temperatures).

The diesel fuel is better for a diesel motor because it self-ignites more readily, and this is desirable in the diesel cycle. If we try to inject gasoline instead of diesel into the diesel engine, this may not work because gasoline, having much higher self-ignition temperature, may not ignite at all. Of course, if we build the diesel engine to have a really, really high compression ratio then even the 120 octane gasoline will self-ignite and our engine will be able to work with almost anything we put in our tank.

(However, remember that the diesel fuel is better in lubricating things than the gasoline, and diesel-engine manufacturers use this property heavily – so using even low-octane gasoline in your diesel engine may fail because fuel supplying parts will not be lubricated enough. Don’t kill your engine!)

How about using diesel in a gasoline engine? Maybe it would be possible to build an Otto-cycle engine that runs on diesel fuel, but it would have to have very low compression ratio to avoid diesel-air mixture self ignition problems. In turn, such an engine would have very poor efficiency (the efficiency is directly connected to the compression ratio). There is additional problem with diesel – it doesn’t vaporize so readily as the gasoline and because of it, such an engine would have to use complicated system of high-pressure injection nozzles to generate rich-enough fuel-air mixtures (as in the diesel engine).

Yes, the diesel engine has benefits (high efficiency, lower fuel restrictions), but it is much more complicated to build than the gasoline engine. The gasoline engine has spark plugs to ignite gasoline-air mixture while the diesel engine needs to have complicated system of high pressure injection nozzles that need to inject controlled amount of fine fuel mist into the cylinder at exactly right moment – a quite difficult job (luckily, the diesel fuel is quite lubricating and non-abrasive so it is not an impossible job.)

The diesel engine is much heavier than the gasoline engine – this is because the higher compression ratio produces higher stress on materials. In the diesel engine you will find thick cylinders, heavy pistons, rods and valves. All this moving mass restricts the speed a diesel engine can turn. A typical car diesel engine doesn’t turn faster than 4000rpm, while a gasoline engine goes up to 7000rpm. (Also notice that the diesel fuel vaporizes quite slowly and, at high speed, there won't be enough time for all the fuel to burn out, thus diesel engine efficiency will be reduced.)

Comparative view of the otto and diesel cycle. In the diesel cycle only the pure air is present in the compression phase, while the fuel is injected just at the beginning of the combustion stroke.

As a side-reading, you can check this interesting text about an old dual-fuel tractor engine. There you can learn about water-injection technology that is sometimes used to solve self-ignition problems.

Controlling power

There is one profound difference between diesel and gasoline engine. This is about the amount of air that comes into the cylinder during the first phase of the cycle (intake).

In the gasoline engine, there is a special valve (throttle) that restricts the amount of air-fuel mixture that is filled into the cylinder. When the engine is idling, only a limited amount of air-fuel mixture is allowed in. At full power, the valve is fully opened allowing the cylinder to suck the maximum amount of the air-fuel mixture in. It means that the final pressure level at the end of compression stroke highly varies depending on how much air-fuel mixture was allowed into cylinder during the intake stroke.

Also, note that the concentration of the fuel in the air-fuel mixture is always the same, no mater what is the power level of the engine. Thus, the mixture is always aimed at full combustion (or more or less so – read further about it). The concentration of fuel in this mixture is, of course, always high enough to make self-sustained burning.

The diesel engine is another story. Here, the amount of air sucked into the cylinder is always the same, invariable of the power level. However the power is controlled by the amount of the fuel that is injected into the cylinder. The good thing is that the final pressure of the air (just before the fuel is injected) is always the same and so the injected fuel will find nice temperature to burn even if the engine is running at low power level. (Even if a very small amount of fuel is injected – smaller than otherwise needed for self-sustained burning – it will all burn out because the surrounding air temperature is high enough).

Now we know how power is controlled in both engines. So what can we do if we want to get more power?

We can improve efficiency. This is the greenest way. But that is for ladies. What we want is creating a monster.

The only other way is to burn more fuel in the same amount of time. The fuel is where the power comes from and if we want power we have to burn it – burn it fast. However, not only that we have to increase supply of fuel, but we also have to increase supply of air. Supply of air is the major problem (the air is bulky) and there are several ways to do it.

The first one is to make larger cylinders or to increase number of cylinders. So, in each cycle more air will come in, and we will be able to burn more fuel producing more power in each engine cycle. This approach works well for both, diesel and gasoline engines.

Second, we can put more air into the cylinder than it is cylinder’s volume – we simply force it inside by turbo-charging or super-charging. If we force two liters of air into one litter cylinder we could get twice the power in each stroke. But of course, as we already started with pressurized air, at the end of the compression stroke we will end up at much higher pressure-level than in non-force-charged engine. In the gasoline engine this will cause premature self-ignition of the air-fuel mixture and our engine will break down (except if we use a high-octane fuel that won’t self-ignite). The diesel engine, however, doesn’t have this problem. That is why turbo-charging or super-charging is more popular way for boosting power on diesel engines than in gasoline engines. (In addition, the diesel engine is by its nature built more robustly than gasoline so increased pressure won’t generate major problems to materials.)

Third way for getting more power would be to make the engine turn faster. Although the same amount of fuel will be burnt in each cycle, the number of cycles in unit of time will increase and thus we get more power. Unfortunately, diesel engines are very limited because of its heavy high-pressure-proof design. Increasing turning speed of a diesel engine would break the engine apart (except if you use stronger, more expensive materials – but there are not many things better than steel). However, the gasoline engine is much lighter – every shaft, cam or rod inside a gasoline engine is light and can move very fast without self-breaking. Thus increasing turning speed is the preferred method of gaining power in the gasoline engine.

Emission control

When a pure hydrocarbon burns in oxygen only water and carbon dioxide is produced. Unfortunately, in the real life things are more complicated.

Neither gasoline nor diesel are pure and they both contain considerable amounts of impurities. Especially diesel is known for its high impurity level (maybe partially because the diesel engine can cope with quite low-refined fuels so no high purification is needed in the first place).

Even more bad stuff happens when a hydrocarbon burns in the air (nitrogen and oxygen mixture). If the combustion takes its place at too high temperature the nitrogen will react with oxygen producing various nitro-oxides that are nasty to environment. So, in the internal-combustion engine it is essential to control the combustion temperature at the optimum level.

In the gasoline engine it is fairly easy. As we told already, the concentration of the air-fuel mixture is always the same (no mater of the power level of the engine) and so we only need to keep this concentration somewhat below optimal to decrease the combustion temperature. This ensures for clean exhaust.

The diesel engine is more problematic. As the air-fuel concentration varies greatly depending on the power level of the engine, we cannot easily control the combustion temperature. Various tricks are used – like restricting the combustion area to only a part of the cylinder or injecting the fuel in several delayed intervals… One regularly used trick seems very charming – the EGR (Exhaust Gas Recirculation):

The combustion temperature will be lowered if a part of oxygen in the air is replaced with some inert gas. To decrease the concentration of the oxygen, some of the engine exhaust (concentrated with inert gases; carbon-dioxyde and water wapor) is supplied back at engine air input. This controls the peek combustion temperature, but also reduces the amount of excesive oxygen available for combining with nitrogen. The EGR is very useful, and is used on both, diesel and gasoline engines.

(Many of my friends with gasoline-powerd cars are puzzled with the questions "why do they measure oxygen (O2) at madatory emission tests?" and "why is it bad to have high oxygen level in the exhaust?". The fact is, as we discussed above, that in the gasoline engine the oxygen to fuel ratio is always almost-ideal and aimed for full combustion, invariable to the throtle level. Therefore, if there is much oxygen left, it is a strong indication that the engine is sub-optimaly tuned.)

Bonus explanation: Why is diesel engine more efficient than gasoline

Is it because of a higher compression? Well, yes, it is, but only indirectly... read below.

Some amount of heat energy will be “generated” during combustion (aka expansion, aka power) stroke in an engine. But not all of the “generated” energy will be transformed to mechanical form. Large part of the heat energy (maybe more than half) will be expelled through the exhaust pipe and therefore lost. Because it uses higher compression ratio, the diesel engine will be able to extract more mechanical energy from heat (that is, the diesel will attain higher efficiency).

If you look at the engine in largely simplified way, you can see that the combustion stroke is a simple adiabatic process.

During the combustion stroke, you start with high pressure (P1) and high temperature (T1) of the combustion gas in the cylinder. Then the piston travels downward for some length to its bottom position, and now you have lower pressure (P2) and lower temperature (T2) of the gas. Obviously, the energy contained in gas is now lower (it has lower temperature and lower pressure, while its molar amount is the same). This difference is converted to mechanical energy of piston.

(You can also look at the above process following way: at the beginning of the expansion stroke, the gas is compressed, hot and energetic and it pushes the piston downward forcefully. Doing this, the gas becomes more and more ‘tired’ - colder and depressurized – because it gives its energy to the piston. Longer it pushes the piston, more of its energy is given to the piston, but at the same time the gas becomes more and more ‘tired’ and pushes the piston less and less ‘enthusiastically’ – that is, with weaker force. After some time you simply have to give up and let the exhausted gas out because it would take the entire eternity for it to give all of its energy to the piston.)

Expansion (combustion) stroke of an gasoline engine - piston in top and bottom position. The gas inside colds down as its volume increases and gives its energy to the piston.
(K-constant, Cp-molar specific heat for constant pressure, Cv-molar specific heat for constant volume)

Now, clearly it is possible to convert more heat energy to mechanical if the difference between P1,T1 and P2,T2 is larger. The equation in the picture above also tells the same – the difference between volumes V1 and V2 is what gives us the mechanical work (energy). That is why diesel beats gasoline – the compression ratio in diesel engine is larger, therefore the volume difference between top and bottom position is also larger, therefore more mechanical work is extracted from heat during combustion (expansion) stroke.

But, during the compression stroke the diesel spends more energy for compression than gasoline? Yes, but it doesn’t matter overall. All that energy that was put into the gas during compression stroke will be recovered during expansion stroke (plus the energy that comes from fuel). Imagine an engine that simply doesn’t fire the fuel – as the engine turns it neither spends nor produces energy. During compression it takes some, but during expansion it gives the same amount. No matter what the compression ratio is. The higher compression ratio of the diesel really doesn’t cost the energy (in first approximation).

Why then there are no engines with compression ration 1:zillion? Sure, such an engine would be efficient. In gasoline engine the limitation is because of self-ignition of the gasoline-air mixture. In diesel engines the limitation is only in mechanical strength of materials (pistons, cylinders, valves, rods, crankshaft...).

What does engine power has to do with efficiency? Well, not that much. Efficiency and power are largely unconnected. However, in general, a less efficient internal combustion machine can be made more powerful. How come? Well, it takes time for gas to give its energy to the piston and the gas gives it slower and slower as it gets colder. If you need a powerful engine, you don’t have time to waste. You have to expel the ‘half-used’ gas and get the fresh one in quickly.

What is Atkinson-cycle? This is a way to improve inherently low efficiency of a gasoline engine. In gasoline engine you are limited about compression ratio. However if you had read the above text more carefully, you can see that the compression ratio doesn’t really matter – what is important for efficiency is the ‘decompression ratio’. Certainly, in normal gasoline and diesel engines those two are the same. But in Atkinson-cycle, the compression stroke doesn’t start at the bottom point of the piston. No no. The valve only closes when the piston is already half-way up... At the top position, the gasoline-air mixture is compressed to its maximum (actually to the same pressure level as in an ordinary gasoline engine ). But during expansion stroke, it goes all the way to the bottom. However the disadvantage is that only smaller amount of gasoline-air mixture is used in every cycle – smaller than in ordinary gasoline engine – and so the power of Atkinson cycle engine will be lower than the power of Otto-cycle engine of the same volume

Danijel Gorupec, 2006

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