Gas engines - what makes them different from diesel and petrol?

let's talk about some fundamentals of gas engines so that we can all kind of start from the same base now gas engines how are they different to other engines fundamentally they're not really it's it's about the fuel that's used to power them so if you've looked at a gas engine on the inside they look much the same as any other engine and in fact from the outside they also look much the same as any other engine so i've got up here one of these is a diesel and the other one is a gas engine be hard-pressed to tell the difference yeah you know caterpillar for example one of the most popular engines on the market is the caterpillar 3516. well there is a 3516 and a 3516g right the g is the only thing that really denotes the fact that it is a gas engine they're both based on the same architecture and a lot of the same technology now when you get to the top end of course things have changed gas engines tend to be spark ignited versus compression ignited but fundamentally they are almost the same engine right they're packaged in almost the same way and they can be used in the same way right like in a lot of cases the the 3516 series is used for power generation both as a diesel engine and as a gas engine so there's not that many differences between a gas engine versus any other kind of engine versus you know the actual fuel that's being used now the fuel that's being used this is of course a combustion engine so we are taking some kind of carbon-based fuel and we're igniting it now we can talk a little bit about some of the more exotic fuels that are being talked about like hydrogen and ammonia but at the moment we're going to speak specifically about the actual gases all right so what we're doing is we're taking fuel we're adding oxygen and what we get is energy plus exhaust right this is the fundamental equation which underpins all combustion engines but instead of using a fuel like petrol no instead we're using a gas right which is predominantly composed of methane in the case of natural gas but it might have some contaminants and it might have some heavier molecules like propane and butane in it as well right now at a very basic level when we burn methane we are combining methane and oxygen and what we get is energy carbon dioxide and water now that's for a perfect burn right in which every single molecule of methane is consumed it's matched with an adjoining oxygen molecule and you get perfect combustion into carbon dioxide water now of course in practice this doesn't actually happen now one of the things that's really interesting is that when you look across the different flame temperatures right for different fuels what you'll see is a very big difference between gasoline and methane now gasoline and diesel they sort of burn in the same ballpark so this comparison holds true for the difference between gas and diesel as well so what you'll see is that you know gasoline engines tend to burn let's say a flame temperature a little bit above 1000 degrees celsius and for methane it's in that ballpark of about 1600 now that's a pretty big difference right in celsius it's a difference of 50 i get that's not an absolute scale if you want a true percentage difference you have to take it relative to absolute zero so let's say that the difference is you know 35 or in that ballpark it's still a significant difference in temperature and that's one of the huge drivers behind why we need specialized engine oils for gas applications it's largely down to the temperature difference right now let's let's talk a little bit about the technologies that are employed in gas engines so you can sort of split up the technologies into these three different groups you've got two versus four stroke you have turbocharged versus naturally aspirated and we have a lean burn versus stoichiometric now i get on that third row there should technically be three options lean burn stoichiometric and bridge burn but in practice we don't really see that many ridgeburn engines alright so let's talk about the number of strokes most of the engines that you're going to see are four-stroke engines right so the way that they operate is much like any other four-stroke engine right that you would have in a in a petrol or a diesel car okay four-stroke engine far and away um kind of like the the most common let's put it that way now there are two-stroke gas engines what is unique about the gas engine market is that let's say for example in day-to-day use we typically associate two-stroke engines with very very small niche applications things like my lawn mower right is a two-stroke engine or some dirt bikes for example uh two-stroke or um yeah i mean like that's a lot or boating right uh there's a lot of two-stroke two-stroke motors as well they tend to be small compact very simple engines right so we're talking about a lot of like household appliances where you um where really anyone can can kind of maintain and work on them that's not the case when it comes to gas engines the two strokes tend to be much much larger than their four-stroke counterparts so with the if you've ever seen like a cooper bessemer style engine these are the ones that are absolutely huge they're gigantic engines um so there are a lot of two-stroke uh gas engines out there they're just not as as common as the four-stroke variety okay then we also have to go into the differences between turbocharged and naturally aspirated now again here turbocharged engines are far more common than naturally aspirated so why would that be the case we're going back to our equation we had methane plus oxygen gives energy carbon dioxide and water well to balance that equation out we actually need two oxygens and two waters and this is an equation which we would call stoichiometric now that's a fancy word that basically means everything is perfectly balanced thanos would approve so what we're basically saying is that there are exactly the correct number of molecules on the left hand side of the equation to make exactly the correct number of molecules on the right hand side of the equation so if somehow i were to create an engine which had one methane molecule in it and two oxygen molecules in it then when i burnt them because they're all in contact with each other i get one carbon dioxide and two waters and then i could multiply that by an integer number so let's say for example i could have four methanes and eight oxygens would give me energy plus four carbon dioxides and eight waters now this is what we call stoichiometric the thing is though we're not putting oxygen into our engine we're putting air into our engine and air is of course a mixture of all kinds of different gases it's predominantly nitrogen then we have a bit of oxygen as well as argon carbon dioxide and a whole bunch of other gases right now how much of air is oxygen it's about 20 i think it's actually like 19.6 or something like that but for our purposes let's say it's about 20 so in order to react methane with two molecules of oxygen right what we actually need is about 10 molecules of air right we say in in that ballpark rate when i say molecules of air that doesn't really make sense does it so it's like 10 units of air i'm picking an arbitrary number here the reason i say that is because about 20 of air is oxygen now when we actually do the calculations right because air is made up of molecules of all sorts of different sizes what you actually get is a ratio of air to fuel which is 16.09 so this would be an air fuel ratio which is perfectly stoichiometric in that ratio there is the exactly correct amount of oxygen to perfectly react with the methane now how does that affect combustion all right when we look at it on on a graph basically what happens is if we've got the air fuel ratio is on the on the x-axis then at some point at 16.09 we call that stoichiometric now if we are to the left of the curve you'll see what happens as we increase the air to fuel ratio we get a dramatic decrease in the amount of carbon monoxide in the exhaust right so remember if we're not getting perfectly stoichiometric combustion then we have all kinds of other weird things that happen in the exhaust a similar thing happens with nox right so we have very little knox production and then near stoichiometric we have a lot of nox production which we don't like and then as we increase the air fuel ratio that nox production decreases again similar thing goes on with oxygen right where the amount of oxygen is very low initially but as we increase the air fuel ratio that means that there is more oxygen than is is needed for combustion which means that we're going to have oxygen in the products right so if i am on the left side of this this is what is called rich burn so i have more fuel than i need so generally what we get is more power on the right hand side this is called lean burn and it generally results in fewer emissions right so if i have oxygen in the exhaust i don't really care although it's a bit of a waste but if i have less carbon monoxide and less knox emissions then that's favorable right so we tend to have because of emissions regulations lean burn engines that's why they tend to be the most popular now one of the things is that in an effort to reduce emissions to the greatest extent we have gone with very very lean burn engines with very high air fuel ratios and the problem with that is and in some cases it's so lean that the spark can't actually ignite that air and fuel mixture so a lot of the time what you'll see is designs where they have what's called a pre-combustion chamber so there's a little chamber up above where it has a bit of a mix with a rich it's a rich burn environment now what that'll enable is the spark to then propagate a flame and once that flame is at a higher temperature it will then burn the lean mixture so we kind of get the best of both worlds all right so that is why in the vast majority of gas engines that you will see today most of them are four-stroke turbocharged lean burn engines if you found this content useful head on over to lubrication.expert it's a website where there's tons more training courses they're more structured and it's available for about 22 us dollars a month

Channel: Lubrication Explained

Views: 11,487

Keywords: Lubricants, Lubrication, Reliability, Machinery, Engineering, Assets, Maintenance

Id: 7Brwa3D5emY

Length: 10min 41sec (641 seconds)

Published: Sat May 28 2022