PETROL vs DIESEL Engines - An in-depth COMPARISON

When you look at a gasoline engine and a

diesel

engine, at first glance they look very similar and that makes sense. Because at the end of the day both are internal combustion

engines

. They combust internally. Which results in them having a very similar anatomy. Both gasoline and

diesel

engines

have a cylinder block and cylinder head, some camshafts, some valves, some pistons, a crankshaft, and some connecting rods. The anatomy is very similar. What is different is the way in which they carry out their combustion. And a more accurate term to describe gasoline and diesel engines is: spark ignition.

And compression ignition, engines. Currently, all commercially available gasoline engines are spark ignition engines. That is, they have a spark plug in the combustion chamber. And what the spark plug does is, well, create a spark. Now, to create a spark, this spark plug needs voltage, and the voltage is supplied by the vehicle's ignition system. In older vehicles, the ignition system consists of a distributor and an igniter. But in newer vehicles, the ignition system does not have a distributor. And instead we have coil packs or a coil-on-plug ignition system. But regardless of the type of ignition system, they all work the same way.

As we said, they provide tension to the spark plug. High voltage usually greater than 20,000 volts. And what this voltage does is it starts to ionize the gases or the air-fuel mixture in the spark channel. And what this does is convert gases from electrical insulators to electrical conductors. Allowing a stream of electrons to arc from the main electrode of the spark plug to the side or ground electrode of the spark plug. This stream of electrons greatly increases the heat in the spark channel. And it causes the gases there to expand rapidly outward. This starts a chain reaction known as a deflagration.

Now, deflagration is a bit of a strange word, but what you need to know about it is that it is a combustion process that occurs subsonically through heat transfer. Which means that the first layer of hot gas transfers heat to the next cold layer, and so on, until all the gases are burned. From an engineering perspective, deflagration is good and desirable. Because it is quite uniform and controllable. And it is ideal for putting an object in motion. For example, a piston in the diameter of a cylinder. Now let's see how a diesel starts combustion. Well, in a diesel engine there are no spark plugs.

No spark, no ignition coils, none of that. A diesel starts combustion by simply compressing the air. The air in a diesel engine is compressed much more than in a gasoline engine. And as we know, as air becomes more and more compressed, it also heats up. And when the piston of a diesel engine reaches top dead center, the air becomes so hot, due to compression, that it is hot enough to ignite the diesel fuel as it is introduced into the combustion chamber. And that is why a diesel engine is also called a compression ignition engine. What creates ignition is the compression of air.

And what is different is not only the way in which combustion starts, but also the moment of introduction of the fuel into the combustion chamber. A gasoline engine introduces fuel before a diesel engine. During the engine intake stroke. And the fuel and air inside a gasoline engine are already well mixed before the spark plug fires. In

comparison

, a diesel introduces the fuel much later. During the engine compression stroke. Fuel can only be introduced when the air is hot enough to ignite the fuel. And of course, air is warmer when it is more compressed. And that's why diesel only introduces fuel very late in the compression stroke, just before the piston reaches top dead center.

Unlike gasoline engines, which can have port injection, direct injection or both. A modern diesel engine almost always only has direct injection. Which means that the fuel is injected directly into the compression chamber through some type of nozzle. Or more commonly, an injector. And when a diesel injector fires, it sends a mist of very small fuel droplets into the combustion chamber, which is now filled with very hot, highly compressed air. This is the beginning of the injection process. When fuel droplets encounter hot air, they begin to vaporize and mix with it. The piston continues to move toward top dead center, further compressing the air.

Which now reaches a temperature high enough to ignite the vaporized fuel. At this point, rapid ignition of the small amount of premixed fuel and air occurs. The time between the start of injection and the start of ignition is called "ignition delay" in a diesel engine. The rapid ignition that occurs at the end of the ignition delay creates a sharp spike in cylinder pressure. This rapid ignition of the initial premixed fuel and the sudden increase in combustion pressure is what generates the characteristic knock or rattle of diesel. This initial part of the combustion process is also usually uncontrolled combustion because combustion can begin at one or more points in the chamber.

As injection continues, new fuel is sprayed into an already burning mixture. Which dramatically increases the rate of vaporization and this fuel begins to burn almost instantly after leaving the nozzle. Because the uncontrolled combustion stage has raised the temperature enough and created enough turbulence, that combustion in the next stage can be controlled by controlling the injection. This part of the combustion process is also called controlled combustion. Once the injection process is complete, some molecules that fail to create a combustible mixture will remain unburned in the chamber. These less volatile molecules will burn as the piston moves down the bore.

And most will have been burned by the time the piston reaches bottom dead center. Now that we have seen the differences in the combustion process of gasoline and diesel engines, let's see how these differences result in these two different engines having very different natures and being good at different things. Now, one of the key differences between gasoline and diesel engines can be observed without even looking at the engine. All you have to do is sit inside the car and look at the tachometer. Gasoline and diesel engines have very different red lines. Most diesel engines reach between 4500 and 5000 RPM.

Compared to this, gasoline engines rev much higher, between 6000 and 9000 RPM. Now, there are three main reasons why diesels don't accelerate as much. The first reason, and the easiest to understand, is that diesel engines require heavy, robust and strong internal components. They need strong internal components because they experience much higher combustion pressures compared to gasoline engines. What's especially important are those combustion pressure spikes that occur during the uncontrolled combustion process we just covered. If the internal parts of the engine were fragile, they would simply fall apart during the process of uncontrolled combustion. That is why diesel engines need strong and therefore heavy internal components.

But heavy internals come at a price. They are harder to turn and limit maximum RPM. The second reason why diesel engines don't rev as much is that most diesel engines are undersized in design. Which means they have a much longer stroke compared to their diameter. And although this is good, because it allows diesels to generate a lot of torque at the beginning of the rev range. It also limits the maximum RPM potential. But the third, and perhaps most important, reason why diesel engines don't rev as much is that they can't really control ignition timing. A gasoline engine that has a spark plug, an ECU, and an ignition coil can fire the spark plug at any point in the engine's compression stroke.

This is called ignition advance. And gasoline engines depend on it to continue generating power at higher speeds. As RPM increases, piston speed also increases. And the spark plug will fire earlier in the compression stroke, to account for the increased piston speed. To make sure the front of the frame starts to spread completely. And that maximum combustion pressure is generated, at the right time, when the piston begins to move downward. Now, a diesel engine cannot do this. You cannot have an ignition timing control range because you cannot start the combustion process before or after the air is fully compressed.

And the air is always completely compressed at the same point in the piston's travel, at top dead center. And although diesel engines do have injection timing. Due to the compression ignition nature of the engine, the injection timing range is much narrower, compared to the ignition advance range of gasoline engines. And that is why diesels usually stop accelerating just at the moment when gasoline engines begin to incorporate increasingly advanced ignition. Now, there is another factor in addition to all this. Diesel fuel burns more slowly than gasoline. And this is another reason why high RPM would essentially be wasted on a diesel.

At high piston speeds, the piston speed would actually be too high. And the slow-spreading flame of the diesel engine would not reach the piston. By the time maximum combustion pressures were built, the piston would be too low in the bore and the diesel would actually make less power at higher RPM. But thinking that not accelerating to the maximum is a failure on the part of the diesel would be a mistake. The diesel does not accelerate much, because it does not need to accelerate very much. It does everything it needs to do, at the low RPM it has.

And while this results in a narrower power band compared to gasoline, it has one very important benefit. Diesels have a long life. Heavy, strong internal components and low revving line help diesel engines last a long time. We all know that engine wear increases exponentially with RPM. And that's why, on average, diesel engines last longer than gasoline engines. When the piston inside your engine is at the bottom of its travel, at the bottom of its stroke, at bottom dead center, that is the largest cylinder volume in your engine. When the piston is at the top, at top dead center, that is the smallest volume of the cylinder.

The ratio, the difference between these two volumes, determines the compression ratio of your engine. And it is a very important number for each engine, because it determines many things for the engine. Now, when it comes to diesel engines, diesels have a much higher compression ratio than gasoline engines. And this results in one of the few very noticeable anatomical differences between gasoline and diesel engines. Here we have a combustion chamber of a gasoline engine. And here we have a combustion chamber, or lack thereof, in a typical modern diesel engine. Here we have the typical piston of a modern gasoline engine.

And here we have the typical piston of a modern diesel engine. As you can see, the smallest volume in a gasoline engine is achieved by the combustion chamber. However, this volume would be too large for diesel. Therefore, it uses a vacuum in the piston to achieve the smallest volume of its cylinder. And of course, the vacuum in the piston is smaller than the combustion chamber, resulting in a smaller volume of diesel. Resulting in a higher compression ratio. Now, a higher compression ratio is better. Because the more the air and fuel are compressed, the smaller the area in which it burns and the greater the force applied to the piston.

To help you visualize why a higher compression ratio is better, you can imagine a stick of dynamite detonated in two different rooms. A small room and a large room. So what do you think? Which room's walls will suffer the most damage from the explosion. Of course, the walls of the small room. Because they are closer to the source of the expanding energy explosion. And the same thing happens in an engine. The closer the piston is to the combustion source, the more force will be exerted on the piston. And that means that with a higher compression ratio, you're making more power.

It also means that your engine is more efficient with a higher compression ratio, because you are harnessing more energy from the same amount of air and fuel. So why don't gasoline engines have the high compression ratios that diesel engines have? Well, they can't. Because they have hit limits. Now, the combustion process withinThe gasoline engine should only be started by the spark plug. And all the air and fuel should burn, just because of the spreading flame front. When combustion occurs spontaneously outside the flame front in a gasoline engine. That's called touching. Knocking sharply increases the combustion pressure within the engine and, if it continues to occur over time, will destroy a gasoline engine.

Therefore, of course, it is not desirable. Now, knock can occur inside a gasoline engine, because the gasoline engine is compressing both air and fuel. And if there is enough compression, or too much, if the temperatures rise enough. The mixture can then spontaneously combust outside the flame front. And that's why gasoline engines must maintain lower compression ratios to account for knock and prevent it from occurring. In

comparison

, a diesel engine only compresses air. There is no fuel, so there is nothing that can spontaneously combust. Inside a diesel engine, fuel is introduced only when combustion is supposed to occur.

This is why knock is not a problem inside a diesel engine. Since they don't have to worry about knock, diesel engines can have a much higher compression ratio. Anywhere between 15:1 and 23:1. In comparison, a modern naturally aspirated gasoline engine typically has a compression ratio between 11:1 and 12:1. Although there are some outliers, running 14:1 compression ratios, they are an exception. And the average is between 11 and 12 to 1. As you can see, much less than a diesel engine. But forced induction gasoline engines have to run at an even lower compression ratio, between 8.5 and 10 to 1, because they have to take into account the fact that the turbocharger, or supercharger, sends already compressed air.

And if you compress it even more, increase its temperature even more, then the chance of a crash increases. And that is why forced induction gasoline engines must take this into account. And make up for it with a slightly lower compression ratio. This is also why tuning forced induction gasoline engines is more challenging than tuning a forced induction diesel and turbocharged diesel. And that's why you can run more boost with less risk in a diesel engine. Now, let's see how all this results in diesel engines generating better torque. And gasoline engines are better at generating horsepower. Now, torque is essentially force applied at a distance.

After all, this is what the units of torque measurement express. When you tighten bolts with a wrench, you are actually applying torque to the bolts. Strength is what you are doing with your hand. And the distance is the length of your key. That's why you use a longer wrench to apply more torque to the bolts. Because you are increasing the distance in the force and distance equation. Resulting in more torque. Now, the same thing is happening inside your engine. Your hand, or force, is actually the combustion process within the engine, which applies force to the piston.

The distance, the length of the wrench, is actually the length of the crank and crank. Now, since diesel engines have more stroke, they need more stroke to compress the air more. In fact, they have more distance in the strength and distance equation. And since they compress the air and fuel more, they generate a more powerful combustion. Which means more strength in the distance and strength equation. And all this results in diesels generating more torque. What is horsepower? Well, horsepower is torque x RPM. Now, gasoline engines generate less torque, but accelerate much faster. And this results in more work done over the same period of time, resulting in gasoline engines making more horsepower.

Gasoline engines typically have a thermal efficiency of between 20% and 35%. Some modern gasoline engines achieve 37 to 38 percent thermal efficiency. A thermal efficiency of 37% means that 37% of the energy generated by combustion is converted into useful work. The rest is simply wasted as heat. In comparison, diesels are more efficient and can achieve thermal efficiency of around 40% to 45%. So why are diesels more efficient? Well, the number one reason is, as we already said, compression. Because diesel engines can compress the air-fuel mixture more, they can extract more energy from the same amount of air and fuel. The other reason why diesels are more efficient is simply because they use less fuel.

Now, the stoichiometric air-fuel ratio for diesel fuel is 14.5 to 1. The stoichiometric air-fuel ratio for gasoline is 14.7 to 1. A stoichiometric air-fuel ratio is an ideal air-fuel ratio, in the one where all the fuel burns. Without leaving any air. So for a 14.7:1 ratio, we need 14.7 units of air. For each unit of fuel. When you increase the amount of air by this ratio, you are creating a lean mixture. When you reduce the amount of air by this ratio, you create a rich mixture. Now, gasoline engines, depending on the load, can operate on both sides of the air-fuel ratio. Normally, at idle and cruising conditions, gasoline engines run with little mixture.

But with full throttle and full load, gasoline engines should run rich. And forced induction gasoline engines should run even richer. Compared to these, diesels are very different. Diesels never get rich. They always run lean. Modern turbocharged diesel engines can achieve an idle air-fuel ratio of around 160:1. As you can see, compared to gasoline, it is extremely lean and there is much more air in diesel than in fuel. And only at full throttle and full load do diesel engines begin to approach a stoichiometric air-fuel ratio. But again, even under full load they are still quite thin. And this means that there is always more air than fuel inside the cylinder of a diesel engine.

And this leads to a difference in how gasoline and diesel engines modulate their power. When you press the accelerator pedal in a gasoline-powered car, you are actually operating the engine's throttle body. The throttle body is a butterfly valve that determines the amount of air that enters the engine. The harder you press the accelerator pedal, the more open the throttle valve is and the more air enters the engine. The engine ECU then uses various sensors to determine, based on the amount of air, how much fuel to inject into the engine. When you press the accelerator pedal in a diesel-equipped vehicle, you are not operating any type of throttle valve.

Instead, you dictate the amount of fuel that enters the engine. Now, don't be confused. Many modern diesel engines have what looks like a throttle body underneath. But when you press the accelerator pedal, you are not operating this throttle valve. The throttle valve in diesel engines is actually used to improve the efficiency of the engine's exhaust gas recirculation system. So why don't gasoline engines run at the same extremely poor air-fuel ratios as diesel engines? Well, gasoline engines can't do this. Because the air and fuel mixture in gasoline engines is homogeneous. In diesel engines the air-fuel mixture is heterogeneous.

A homogeneous air-fuel mixture means that all the air and fuel are mixed evenly. A heterogeneous or stratified air-fuel mixture means that only a portion of the air is mixed with the fuel in the diesel engine. And this allows them to function extremely agile. An extremely poor condition in a gasoline engine is that it runs much hotter and is also more prone to detonation. And that is why gasoline engines should avoid it. But another reason why diesel engines manage to be more efficient than gasoline engines is the fuel itself. Diesel fuel is made up of longer hydrocarbon chains compared to gasoline.

And because of this, it is more energy dense. For the same amount of volume, diesel fuel has about 15% more stored energy. But fuel efficiency is only part of the economic equation. Now, diesel-powered cars, trucks, and other vehicles are typically more expensive to buy when new. They also tend to be more expensive on the used car market. Diesel engines are more complex. Which means they are more expensive to manufacture. Which leads to a higher final price for consumers. Another thing that has been changing for diesel is that awareness about pollution has increased. In addition to things like the diesel scandal, it has pushed for changes to diesel-related legislation and taxes.

Which means that the resale values ​​of diesels have been changing in various markets around the world. In some markets where diesels used to have strong resale values, this has started to change and it is now difficult to rely on the strong resale values ​​of diesels. As a general rule, you have to drive enough for a diesel to make financial sense. You have to travel enough miles to take advantage of the greater fuel efficiency of diesel. Otherwise, if you don't get enough miles, buying gas probably makes more financial sense. Traditionally, emissions from gasoline and diesel engines have been presented as a trade-off between environmentally damaging CO2 and health-damaging nitrogen oxides and soot particles.

Diesels emit less CO2, simply because they use less fuel. But the high-temperature, high-pressure, high-pressure stratified combustion process in diesel actually favors the production of nitrogen oxides and soot particles. Now, things are not really that simple. And they're not really black and white. The fact is that many air quality monitoring agencies around the world, in different countries, have conducted many independent road tests of equivalent vehicles equipped with equivalent gasoline and diesel engines. And almost all of them have found that the gap in CO2 emissions between gasoline and diesel engines is negligible. Typically around 3 to 5 grams of CO2 per kilometer.

And when it comes to nitrogen oxide and soot particle emissions, modern diesel engines, with a Euro 6 or equivalent emissions standard, are quite clean. Because they have very complex emissions equipment. In reality, things like diesel particulate filters and diesel exhaust fluid injection are used to trap or chemically convert the vast majority, more than 90%, of nitrogen oxides and soot particles. Therefore, when new, both gasoline and diesel engines are quite clean, as long as they are properly maintained. In fact, problems begin to arise when cars with diesel and gasoline engines reach the second-hand market. And this is where diesel is at a disadvantage.

Because the emissions equipment of a diesel engine is quite complex. It is also very expensive. And replacing a specific diesel filter in a vehicle that is between 5 and 10 years old can represent between 15 and 30% of the value of the entire vehicle. And this makes it very difficult to justify this replacement for a used diesel vehicle owner. And that's why some owners look for less efficient but cheaper methods to temporarily reduce their emissions, just so they can pass an emissions test and continue driving their vehicles. And an emissions test, after all, only really checks the vehicle once every one or two years.

The remaining 364 days, the vehicle is not actually controlled. And it is free to pollute a lot. Another thing that puts diesel at an advantage when it comes to pollution is that diesel engines are heavier and more complex. Which means they use more materials and more parts to manufacture. And that is why its manufacturing generates more pollution. On the other hand, diesel engines tend to last longer before having to be recycled. Now, fun is something completely subjective. And the reality is that you can have a lot of fun with both a gasoline and a diesel engine. That said, on a narrow and winding road, it is very difficult to oppose a gasoline engine.

It has a wider power band and a better soundtrack. And while it will give you less mpg on a highway like this, it will give you more smiles per gallon. Another thing that works against diesel, when it comes to fun, is weight. Diesel engines are usually quite heavy, usually noticeably heavier than an equivalent gasoline engine. And this, of course, negatively affects the dynamic balance of a vehicle. And that's why, for example, front-wheel drive vehicles equipped with a diesel engine are always going to be more prone to understeer, compared to the same vehicle with an equivalent gasoline engine. but sayThat diesels are not fun would be a big mistake.

Diesels can provide the sensation of enormous torque. And this can be very addictive. In addition, diesels have greater autonomy. Which means they can travel more miles with the same amount of fuel. And this can be very important. And diesels are your best ally, when it comes to long-distance exploration and adventure vehicles. And there you have it. That's all for today's video. I hope this comparison between gasoline and diesel engines helps you better understand how they differ. And why they are different. And how this affects its different behaviors and applications. As always, thank you very much for watching.

I'll see you soon. With more fun and useful things. On channel D4A.