Gasoline Vs Diesel - 4 Major Differences

hello everyone and welcome in this video we're going to be talking about gasoline versus diesel engines and four major differences between them so in front of me i have a single cylinder four-stroke gasoline engine and this will be very helpful in demonstrating how these engines work differently so the first thing we're going to talk about is perhaps one of the biggest differences and that's how they ignite the air fuel mixture so with the gasoline engine you use a spark plug to ignite that air fuel mixture and with a diesel engine you use compression now something you need to understand is called the self ignition temperature and this is the temperature for which an air fuel mixture will combust simply based on the heat without the views of a spark plug and so if you bring an air fuel mixture up to this temperature simply through doing it through compression that air fuel mixture is going to ignite and you don't need an external source to ignite it so the important thing to do with a gasoline engine is to make sure that when you're compressing that air fuel mixture during your compression stroke you don't compress it so much that you exceed that self-ignition temperature and then because of that you're going to have ignition occur before you want it to before the spark plug ignites it now with a diesel engine on the other hand you want to do this differently so you want to make sure that as you compress that air fuel mixture you go beyond that self-ignition temperature so that when you do inject the fuel in after you've compressed the air it's at a temperature high enough so that as soon as that fuel gets injected in you can ignite that fuel and then you have combustion occur so both of them use the same essential four strokes you've got your intake you've got your compression where you compress that air or air fuel mixture you have your fuel shot in with the diesel engine at that point and instead of so instead of that spark plug you're going to have a fuel injector and then in the gasoline engine you have the spark igniting that air fuel mixture so you have your power stroke and then you press that air and fuel out after it's combusted so you press out the exhaust gases so because of that self-ignition temperature and you want to make sure that diesels go above it and the gasoline stay below it this is why you have a major difference in compression ratios between gasoline and diesel engines so gasoline engines will typically have much lower compression ratios somewhere around 10 to 1 versus diesel engines can be much higher 16 to 1 even higher than that and so that's the biggest difference is you want to make sure that you get that air hot enough so that when you spray that fuel in that diesel fuel it ignites it getting into number two throttle versus no throttle typically in a diesel engine you're not going to have a throttle body with this butterfly valve within versus in a gasoline engine you're always going to have this throttle valve and so the reason you have this on a gasoline engine this is how you manipulate how much power you're making so when you press on the accelerator pedal what you're doing is you're opening up this butterfly valve so the more you open it the more air goes into the engine the more air that you have going into the engine the more fuel is injected and then you make more power in a diesel on the other hand what you're doing is actually manipulating the amount of fuel that gets injected in so there's no throttle you just have a pure opening nothing blocking it and so you've always got you know a good amount of air coming in but what you differ is how much fuel you inject so as you inject more fuel you make more power that creates more exhaust that spools up your turbo more and you pull in more air more fuel and you continue to grow more power now some modern diesels will actually have throttle valves in the intake and what that allows you to do is manipulate the pressure within the intake manifold and so by doing that you can increase the amount of exhaust gas recirculation now the other thing that you may do it for to have a throttle valve in a diesel engine is that when you shut the engine off uh you can start to close that valve slowly and so as that engine shuts off it's not a really abrupt harsh thing because you're starting to limit the amount of air that goes into it and so you have a smoother shutdown moving on to number three we're going to talk about air fuel ratios and this part helps explain why diesel engines are capable of creating more power simply by injecting more fuel and so one of the interesting things about gasoline and diesel engines is that they have very similar stoichiometric air fuel ratios so this is the ratio at which all the oxygen and all the fuel is going to be burned and that's somewhere around 14.5 to 15 to 1 depending on the hydrocarbon mixture of that fuel if you're curious about where that number comes from i have a separate video explaining the math behind it which you're welcome to check out i'll have a link in the video description so the interesting thing about this is and here we just have a chart of air fuel ratios from one to one to 75 to one the interesting thing is that although they have the same stoichiometric ratio they operate in vastly different ranges and so a gasoline engine uh combustion can occur from an air fuel ratio of about six to one to about 25 to one but typically engines are going to operate somewhere in the 12 to 18 to 1 uh ratio range a diesel engine on the other hand operates on a much larger from from about 18 to 1 to about 70 to 1 so anything above stoichiometric is going to be lean anything below is going to be considered rich and so as you can see diesels are typically going to operate in this lean range and what you do is you simply inject more fuel and that brings your air fuel mixture down and you create more power and that's bringing you down this scale for the air fuel mixture so how is it possible for diesel engines to operate across such a large range of air fuel mixtures well the important thing to realize is that in a diesel engine when you have direct injection and that diesel fuel is injected in you're going to have pockets of fuel which are combustible because they're within a combustible range and then you're going to have pockets which are either too lean or too rich and so while your air fuel ratio overall may be 65 to 1 some of those pockets are going to be at much lower ratios and so they're going to be able to combust some are going to be too lean some are going to be too rich those won't combust and so it will have to wait until those you know get to the proper ratio for them to combust so your overall ratio may be out here but you'll have some pockets uh which are low enough which they can combust and so you will have combustion occur in that diesel engine all right so moving into our final point we're going to be talking about engine braking so in a gasoline engine the way engine braking works is that when you let off the gas pedal this throttle valve closes and so because that throttle valve is closed whenever you're going on your intake stroke and this piston is moving down and it's trying to pull in air this throttle valve is creating a block so you're creating a vacuum in here and that vacuum is what's going to be slowing your vehicle down so you're it's going to be slowing your engine down to do that your engine is connected to your transmission and thus the wheels and through that the vehicle starts to slow down well in a diesel engine that option's not always available because you're not going to have this throttle valve so you can't create a vacuum between it and the cylinder within the intake manifold so the difference that happens in a diesel engine there's actually two different methods you know for smaller trucks and smaller diesels what they'll do is they'll put a valve in the exhaust so that when you are on your exhaust stroke and you're trying to press out that uh exhaust you're gonna have a valve in your exhaust that's gonna start to close much like a throttle valve and so that's gonna create this back pressure within your exhaust and so that's going to slow down your vehicle because it's trying to push out these gases building up pressure against this piston but it cannot because of that valve so you know obviously it allows some to pass through but it slows your vehicle down now the other method and the one that's used for larger diesel engines and it's commonly referred to as a jake break what you'll have happen is during your compression stroke uh normally if you leave these valves closed if you don't open up either your intake or your exhaust valve during your compression stroke which you wouldn't do all of that pressure that's built up within there then goes to press that piston down so you don't really have much energy lost because though you did have to pressurize that air uh that pressurized air then pushes back and continues uh so you don't really have a net energy loss a little bit but not much so the thing that you want to do is during that compression stroke once it gets up to top dead center you're going to open up the exhaust valve and so when you open up that exhaust valve you're going to let that exhaust air out or that compressed air out and so it no longer is going to be used in reverse to press that piston down so you're working to compress the air and then you're letting that compressed air out and you work to compress that air and then you let that compressor out and that's why you hear it actually is that exhaust valve opening and that compressed air coming out and they can be rather noisy of course the clearance is going to be such that when the exhaust valve opens it's not going to come into contact with the piston once it comes up close to top dead center but essentially what you want to do is let that pressurized air out so that you don't have it press the piston back down and then you don't have much of an energy loss so that's how diesel uh engines uh will engine brake that's how you'll slow down a vehicle using the engine in a diesel vehicle and those are some of the major differences between gasoline and diesel engines so if you have any questions or comments feel free to leave those below thanks for watching

Channel: Engineering Explained

Views: 1,753,908

Keywords: gas vs diesel, gasoline vs diesel, gasoline, petrol vs diesel, compression ratio, efficiency, horsepower, torque, cars, jake brake, how jake brake works, diesel no throttle, how diesels work, how gas engines work, how it works, engineering explained

Id: rXVJG9n6BAs

Length: 9min 26sec (566 seconds)

Published: Wed Mar 09 2016