PETROL vs DIESEL Engines - An in-depth COMPARISON

when you look at a petrol and a diesel engine upon first glance they look very similar and that makes sense because after all both are internal combustion engines they do their combustion internally which results in them having very similar anatomy both petrol and diesel engines have a cylinder block and they sell on their head some camshafts some valves some pistons a crankshaft and some rods the anatomy is very similar what's different is the way they do their combustion and a more accurate term described petrol and diesel engines is actually spark ignition and compression ignition engines [Music] now all commercially available petrol engines are spark ignition engines meaning that they have a spark plug in the combustion chamber and what the spark plug does is well it creates a spark now to create a spark the spark plug needs voltage and the voltage is supplied by the vehicle's ignition system on older vehicles the ignition system consists of a distributor and an igniter but on newer vehicles the ignition system is distributor os and instead we have coil packs or a coil on plug ignition system but regardless of the kind of the ignition system they all work the same way as we said they provide voltage to the spark plug a lot of voltage usually in excess of 20 000 volts and what this voltage does is that it starts ionizing the gases or the air fuel mixture in the spark channel and what this does is that it converts the gases from electrical insulators to electrical conductors allowing a current of electrons to arc from the main electrode to the spark plug to the side or ground electrode of the spark plug this current of electrons intensely raises the heat in the spark channel and causes the gases there to rapidly expand outward this starts a chain reaction known as deflogration now defogration is a bit of a weird word but what you have to know about it is that it's a combustion process that occurs subsonically through heat transfer meaning that the first layer of hot gases transfers the heat to the next cold layer and so on until all of the gases are burned away from an engineering perspective deflagration is good and desirable because it's pretty even and controllable and it's ideal to get an object moving for example a piston in a cylinder bore now let's see how a diesel initiates combustion well in a diesel engine there's no spark plugs no spark no ignition coils none of that a diesel initiates combustion purely by compressing the air the air in a diesel engine gets compressed to a much higher degree than in a petrol engine and as we know as the air gets compressed more and more it also heats up and as the piston in a diesel engine reaches top dead center the air gets so hot due to the compression that it's hot enough to ignite the diesel fuel as it's introduced to the combustion chamber and this is why a diesel engine is also called a compression ignition engine what creates ignition is the compression of the air and what's different isn't just the way combustion is initiated it's also the timing of the introduction of the fuel into the combustion chamber a petrol engine introduces fuel earlier than a diesel during the intake stroke of the engine and the fuel and air inside a petrol engine are already properly mixed before the spark plug fires compared to this a diesel introduces fuel much later during the compression stroke of the engine fuel can only be introduced when the air is hot enough to ignite the fuel and of course the air is hottest when it's most compressed and this is why the diesel only introduces fuel very late in the compression stroke just before the piston hits top dead center unlike petrol engines which can feature port injection or direct injection or both a modern diesel engine almost always features only direct injection meaning that fuel is directly injected into the combustion chamber using some form of nozzle or more commonly an injector and when a diesel injector fires it sends a mist of very small droplets of fuel into the combustion chamber that is now filled with highly compressed very hot air this is the beginning of the injection process as the fuel droplets meet with the hot air they start to vaporize and mix with it the piston continues to travel towards 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 the injection and the start of the ignition is called the ignition delay in a diesel engine the rapid ignition that occurs at the end of the ignition delay creates a sharp cylinder pressure spike this rapid ignition of the initial premixed fuel and the sudden increase in the combustion pressure is what generates the characteristic knocking or clattering sound of the diesel this initial part of the combustion process is also often called uncontrolled combustion because combustion may start at any one or multiple points in the chamber as injection continues the new fuel is sprayed into an already burning mixture which dramatically increases the rate of vaporization and this fuel starts burning almost instantly after it leaves the nozzle because the uncontrolled combustion stage has raised the temperature enough and created enough turbulence that combustion in the following stage can be controlled by controlling the injection this part of the combustion process is also called controlled combustion after the injection process is finished some few molecules that fail to create a combustible mixture will remain unburned in the chamber these less volatile molecules will undergo combustion as the piston travels further down the bore and most will be burned away by the time the piston reaches bottom dead center now that we have seen the differences in the combustion process of petrol 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 petrol 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 petrol and diesel engines have very different red lines most diesels rev to anywhere between four and a half to five thousand rpm compared to this petrol engines rev much higher anywhere between 6 and 9 000 rpm now there's three main reasons why diesels don't rent as high the first reason and the one that's easiest to understand is that diesel engines require heavy beefy strong internals they need strong internals because they experience much higher combustion pressures compared to petrol engines what's especially important are those combustion pressure spikes that occur during the uncontrolled combustion process we just cover if the engine internals were fragile they would simply fall apart during the uncontrolled combustion process this is why diesels need strong and therefore heavy internals but heavy internals have a price they are harder to spin and they limit the max rpm the second reason the reason why diesels don't drive as high is that most diesel engines are under square by design meaning that they have a much longer stroke compared to their bore and although this is good because it allows diesels to make a lot of torque early in the rev range it also too limits max rpm potential but the third and perhaps most important reasons why diesels don't drive as high is that they can't really control uh ignition timing a petrol engine that has a spark plug and an ecu and an ignition coil can fire the spark plug at any point in the compression stroke of the engine this is called ignition advance and petrol engines rely on it to keep making power at higher engine speeds as the rpm increases so too does the speed of the piston and the spark plug is going to fire earlier in the compression stroke to account for the increased speed of the piston to make sure that the flame front starts fully propagating and that maximum combustion pressure is generated at the right time when the piston starts moving down the bore now a diesel engine can't do this it cannot have a wide range of ignition timing control because it cannot start the combustion event sooner or later than when the air is fully compressed and the air is always fully compressed at the same point in the pistons travel atop dead center and although diesel engines do have injection timing due to the compression ignition nature of the engine the range of injection timing is much much more narrow compared to the range of the ignition advance of petrol engines and this is why diesels usually stop revving right at the point where petrol engines start incorporating more and more ignition advance now there's another factor on top of all of this and that's the diesel fuel burns slower than petrol fuel and this is another reason why high rpms would essentially be wasted on a diesel at high piston speeds the piston speed would actually be too high and the slowly propagating frame front of the diesel would miss the piston by the time maximum combustion pressures were generated the piston would be too far down the bore and the diesel would actually make less power at higher rpm but thinking that not revving high is a failure on part of the diesel would be wrong the diesel doesn't rev high because it doesn't need to rev high it does everything it needs to do at the low rpms it has and although this does result in a more narrow power band compared to the petrol it does have a very important benefit diesels are long lived the heavy internals strong internals and the low redline help diesels last a long time we all know that engine wear exponentially increases with rpm and this is why on average diesel engines last longer than petrol ones when the piston inside your engine is at the bottom of its travel at the bottom of its stroke at bottom dead center that's the largest cylinder volume of your engine when your piston is at the top at the top that's center that's the smallest volume of your cylinder the ratio the difference between these two volumes determines the compression ratio of your engine and it's a very important number for every engine and because it determines a lot of things for the engine now when it comes to diesels diesels have a much higher compression ratio than petrol engines and this results in one of the few very noticeable anatomy differences between petrol and diesel engines here we have a combustion chamber of a petrol engine and here we have a combustion chamber or lack thereof in a typical modern diesel engine here we have a typical modern petrol engine piston and here we have a typical modern diesel engine piston as you can see the smallest volume in a petrol engine is achieved by the combustion chamber however this volume would be too large for the diesel so it uses instead a void in the piston to achieve the smallest volume of its cylinder and of course the void in the piston is smaller than the combustion chamber leading to a smaller smallest volume of a diesel which results in a higher compression ratio now a higher compression ratio is better because the more you compress the air in fuel the smaller the area in which it combusts the higher the force applied onto the piston to help you visualize why a higher compression ratio is better you can imagine a stick of dynamite being detonated in two different rooms one small room and one large room so what do you think the walls of which room are going to see more damage from the explosion of course the walls in the small room because they're closer to the source of the expanding explosion of the energy and the same thing happens in an engine the closer the piston to the source of the combustion the more force is going to be exerted on top of 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're harnessing more energy from the same amount of air and fuel so why don't petrol engines run the high compression ratios that diesel engines have well they can't because they're not limited now the combustion process inside the petrol engine should only be initiated by the spark plug and all the air and fuel should be burned away only by the propagating flame front when combustion spontaneously occurs outside the fine front in a petrol engine that's called knock knock sharply raises the combustion pressures inside the engine and if it continues to occur over time it will destroy a petrol engine this is why of course it's undesirable now knock can occur inside a petrol engine because the petrol engine is compressing both air and fuel and if there's enough compression or too much of it if the temperatures get high enough then then the mixture can spontaneously combust outside the flame front and this is why petrol engines must maintain a lower compression ratios to account for knock and to prevent it from happening compared to this a diesel engine is compressing only air there's no fuel so there's nothing that can spontaneously combust inside a diesel engine fuel is introduced only when combustion is supposed to happen this is why knock isn't an issue inside a diesel engine because they don't have to worry about knock diesels can run a much higher compression ratio anywhere between 15 to 1 to 23 to t1 compared to this a modern naturally aspirated petrol engine usually has a compression ratio between 11 uh to 12 to 1. although there are some outliers that run at 14 to 1 compression ratios they are an exception and the average is between 11 to 12 to one as you can see much much lower than a diesel engine but forest induction petrol engines have to run an even lower compression ratio between eight to five to ten to one because they have to account for the fact that the turbocharger or supercharger is sending in already compressed air and if you compress it even further increase its temperature even further then the chance for knock increases and this is why forced induction petrol engines have to account for this and compensate with a slightly lower compression ratio this is also why tuning forced induction petrol engines is more challenging than tuning a forced induction diesel a turbocharged diesel and this is why you can run more boost with less risk in a diesel engine now let's see how all of this results in diesel engines being better at making torque and petrol engines being better at making horsepower now torque is essentially force applied at a distance this is what the measurement units for torque express after all when you're tightening down bolts with a wrench you're actually applying torque to the bolts the force is what you're doing with your hand and the distance is the length of your wrench this is why you use a longer wrench to apply more torque to the bolts because you're increasing the distance in the force and distance equation resulting in more torque now the same thing is actually happening inside your engine your hand or the force is actually the combustion process inside the engine which applies force on top of the piston the distance the length of the wrench is actually the length of the connecting rod and the crank throws now because diesel engines have more stroke they need more stroke to compress the airborne they actually have more distance in in the force and distance equation and because they compress the air and fuel more they generate more powerful combustion which means more force in the distance and force equation and all this results in diesels making more torque what is horsepower well horsepower is torque multiplied by rpm now petrol engines make less torque but they rev higher much higher and this results in more work done over the same period of time which results in petrol engines making more horsepower petrol engines typically have a thermal efficiency between 20 and 35 percent with some modern petrol engines managing around 37 to 38 percent thermal efficiency a 37 thermal efficiency means that 37 of the energy generated by combustion is turned into useful work the rest is simply wasted as heat compared to this diesels are more efficient and can manage a thermal efficiency of around 40 to 45 so why are diesels more efficient well the reason number one is as we already said compression because diesels are able to compress the air fuel mixture more they can squeeze more energy out of 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 one the stoichiometric air fuel ratio for petrol fuel is 14.71 a stoichiometric air fuel ratio is an ideal air fuel ratio at which all the fuel is burned away with none of the air left behind so for a ratio of 14.721 we need 14.7 units of air for every single unit of fuel when you increase the amount of air in this ratio you're creating a lean mixture when you reduce the amount of air in this ratio you're creating a rich mixture now petrol engines depending on the load can run on both sides of the air fuel ratio typically at idle and at cruise conditions petrol engines run lean but at wide open throttle at full load petrol engines must run rich and forced induction petrol engines need to run even richer compared to this diesels are very different diesels never run rich they always run lean modern turbocharged diesel engines can manage an idle air fuel ratio of around 160 to 1. as you can see compared to petrol this is extremely lean and there's a lot more air in the diesel than fuel and only at wide open throttle at full load the diesel sort of start to approach a stoichiometric air fuel ratio but again even at full world they are still pretty lean and this means that inside a diesel engine cylinder there's always more air than fuel and this leads to a difference in how petrol and diesel engines modulate their power when you press down on the throttle pedal in a petrol engine car you're actually operating the engine's throttle body the throttle body is a butterfly valve which dictates how much air is coming into the engine the more you press on the throttle pedal the more the butterfly valve is open and the more air is coming into the engine then the engine's ecu uses various sensors to determine based on the amount of air how much fuel should be injected into the engine when you press down the throttle pedal in a diesel diesel-equipped vehicle you're not operating any sort of butterfly valve instead you're dictating the amount of fuel that goes into the engine now don't be confused many modern diesels actually do have something that looks like a throttle body under but when you're pressing on the throttle pedal you're not operating this butterfly valve the butterfly valve on the diesels is actually used to improve the efficiency of the engine's exhaust gas recirculation system so why don't petrol engines run with these same extremely lean uh air fuel ratios as diesel engines do well petrol engines can't do this because the air fuel mixture in petrol engines is homogeneous in diesel engines the air fuel mixture is heterogeneous a homogeneous air fuel mixture means that all of the air and fuel is evenly mixed a heterogeneous or a stratified air fuel mixture means that only one part of the air is mixing with the fuel in the diesel engine and this allows them to run extremely lean an extremely lean condition in a petrol engine is running a lot hotter and is also more prone to knock and this is why petrol engines must avoid it but another reason why diesel engines manage to be more efficient than petrol engines is the fuel itself diesel fuel is composed of longer chains of hydrocarbons compared to petrol fuel and because of this it is more energy dense for the same amount of volume diesel fuel has around 15 percent more energy stored in it but fuel efficiency is only one part of the economy equation now diesel engine cars and the light trucks and other vehicles are typically more expensive to purchase when new they're also usually more expensive in the used car market diesel engines are more complex which means they're more expensive to manufacture which leads to a higher end price for consumers another thing that has been changing for the diesel is that increased pollution awareness as well as stuff like the dieselgate scandal has been driving change in diesel-related taxation and legislation which means it has been changing the resale values of diesels in various markets around the world in some markets where diesels used to have a strong resale value this has started changing and it's now hard to rely on the strong resale values of diesels as a typical rule of thumb you have to drive enough for a diesel to make financial sense you have to cover enough miles to harness the increased fuel efficiency of the diesel otherwise if you're not covering enough miles a petrol is likely going to make more financial sense traditionally the emissions of petrol and diesel engines have been presented as a trade-off between environment harming co2 and health harming nitrogen oxides and soot particles diesels emit less co2 simply because they use less fuel but the high temperature high pressure stratified combustion process in the diesel actually favors the production of nitrogen oxides and soot particles now things aren't actually that simple and they aren't really black and white fact is that many air quality monitoring agencies across the world in different countries have done many independent road tests of equivalent vehicles equipped with equivalent petrol and diesel engines and almost all of them have found that the co2 emissions gap between petrol and diesel engines is negligible usually around 3 to 5 grams of co2 per kilometer and when it comes to nitrogen oxide and soot particle emissions modern diesel engines of a euro 6 or equivalent emission standard are actually pretty clean because they have some very complex emissions equipment stuff like diesel particulate filters and diesel exhaust fluid injection are actually used to either trap or chemically convert the vast majority more than 90 percent of the nitrogen oxides and soot particles so when they're new both petrol and diesel engines are pretty clean as long as they're maintained properly problems actually start to arise when diesel and petrol engine cars hit the used market and this is where the diesel is at a disadvantage because the emissions equipment of a diesel engine is pretty complex it's also very expensive and the replacement of a diesel particulate filter on a vehicle that's 5 to 10 years old can amount to as much as 15 to 30 percent of the value of the entire vehicle and this makes it very hard to justify this replacement for an owner of a used diesel vehicles and this is why some owners look to less efficient but cheaper methods to temporarily lower their emissions just so they could pass an emissions test and continue driving their vehicles and an emission test at the end of the day actually monitors the vehicle only once every one or two years the remaining 364 days the vehicle isn't really monitored and it's free to do quite a bit of pollution something else that puts the diesel at a disadvantage when it comes to pollution is that diesel engines are heavier and more complex meaning that they use more materials and more parts to be made and this is why their manufacture creates more pollution on the other hand diesel engines typically last longer before they need to be recycled now fun is an entirely subjective thing and the reality is that a lot of fun can be had with both a petrol and a diesel engine that being said on a tight twisty road it's very hard to argue against a petrol engine it has a wider power band and a better soundtrack and although it will give you less miles per gallon on a road like this is going to give you more smiles per gallon another thing that works against the diesel when it comes to fun is weight diesel engines are typically pretty heavy usually noticeably heavier than an equivalent petrol engine and this of course negatively impacts the dynamic balance of a vehicle and this is 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 petrol engine but saying that diesels aren't fun would be very wrong diesels can provide the sensation of massive massive torque and this can be very addictive on top of this diesels have greater autonomy which means they can do more miles with the same amount of fuel and this can be very important and this is why diesels are your best ally when it comes to long distance adventure and exploration vehicles and there you have it that's pretty much it for today's video i hope this comparison between petrol and diesel engines helps you better understand how they're different why they're different and how this impacts their different behavior and application as always thanks a lot for watching i'll be seeing you soon with more fun and useful stuff on the d4a channel

Channel: driving 4 answers

Views: 1,154,639

Rating: 4.9352899 out of 5

Keywords: petrol, diesel, diesel power, petrol vs diesel, petrol vs diesel car, petrol vs diesel engine, petrol vs diesel which is best, petrol vs diesel power, petrol vs diesel which is better, petrol vs diesel performance, petrol vs diesel environment, engine compression ratio, engine air fuel ratio, petrol vs diesel emissions, dieselgate, vw diesel, diesel injectors, diesel dpf, diesel adblue, spark ignition, compression ignition engine, diesel power torque, petrol power torque

Id: aWeqyAxlM2M

Length: 26min 37sec (1597 seconds)

Published: Sun Oct 18 2020