How does a Detroit Diesel two stroke work?

hey they thank us to hit today's video is about how our Detroit Diesel two-stroke works and is proudly sponsored by marine engine calm as you know from the last video I had a bit of trouble with the blower on my Detroit for someone and really fortunately a mate of mine Andrew had one who didn't need so I've swapped to that Detroit blower for this Honda our board how these Detroit Diesel's actually work as a question I get asked a lot and rather than answering and comments or thought been off doing a video on it while waiting for the blower to get fixed what we'll do first though is have a look at how our four-stroke works and how our more traditional petrol to throat works so we've got same to compare it to it straight off the bat one of the things that people use to identify whether an outboard is a two-stroke or four-stroke is see where those got valves or not the valve sitting at the top here if it's got valves a four-stroke if it doesn't stir to struggle the confusing thing is that the Detroit Diesel two-stroke has valves but we'll get to that later so really quickly I know you guys probably know this but the four cycles of a four-stroke motor intake where an intake valve opens the piston comes down draws fresh air in then that all the valves close the intake valve closes the piston comes up compresses that air some form of fuel gets either injected or pre-mixed into the air if it's a carbureted outboard then it gets ignited with a spark piston comes down on the power stroke then the exhaust valve opens piston goes back up pushes the exhaust out so that's the kind of really quick four-stroke cycle they call the suck squeeze bang blow when it comes to lubrication four-stroke engines have oil sitting in the sump unless it's a remote something that you know generally and this oil gets picked up by an oil pump and put under pressure through all sorts of galleries that end up putting a pressurized film of oil metal they talk about with your oil pressure inside these slipper bearings and that sort of pressurized oil is what reduces the friction if your oil pump fails you'll get no oil pressure also if all these spaces inside the bearings between the con rods and whatever gets too great then it doesn't matter what volume your oil pump pumps you don't end up with oil pressure because there isn't enough back pressure these clearances are just too great if you run out of oil obviously but oil pressure and if you overfill it you'll get to the point where the crankshaft is actually hitting the surface of the oil and it'll just start whipping it into a bit of a foam that can't be pumped under pressure either so both of those stop your lubrication all right this is a pretty standard diagram of a two-stroke engine slightly different to the outboards we've seen in previous videos but I'll show you how that's different in this case from a lubrication point of view you have petrol air mixture coming in but you've also got oil in the fuel and instead of having slipper bearings you have roller bearings that don't need oil pressure inside them they need oil in them but doesn't need to be under pressure so as this petrol air and oil mixture comes in it sort of works its way into these bearings and lubricates them now we'll start here at the top of power stroke where we've got compressed oil fuel and air inside the top of the piston spark fires it comes down eventually this piston travels down and exposes this exhaust port and that's what allows those gasses to start escaping that's the end of your power stroke what's also happening is that air that's coming to this crankcase is being compressed below the piston head as the piston went up the mixture is sucked into the crankcase because of the vacuum left behind the piston and then as it comes down it compresses it as it comes down the top of the piston here gets past this transfer port which is actually shown over here so the Pistons come down we've fired exhausts going out the bottom of the piston here is compressing the mixture in the crankcase then eventually that mixture flows up into the combustion chamber one of the slight inefficiencies of two strokes is that this under fuel and air mixture can actually just then go straight out the exhaust the combustion chambers are designed to minimize that but it can happen then the piston comes up again refreshes the crankcase compresses the air and fuel here and fires it down and that's how you get your two-stroke cycle oh and I was going to say one of the main differences we see in the two strokes been working on in the past is that right at the back here you have Reed valves and then the carburetor attached to the bottom of the crank so rather than having this port here you actually have one-way Reed valve that allow the air and fuel and oil to get sucked in but don't allow it to be pushed out back through the rebounds in a one-way valve so instead of getting pushed back out through the carburetor it goes through the transfer port if you see fuel and air coming out this way you read valves are bad all right deep breath hope that made sense sorry I can explain that really quickly because this video is about what makes a Detroit Diesel unique so now let's get into that but at a slower pace a little bit more detail okay so this is the principles operation page from the Detroit Diesel manual it's broken up into the four kind of stages but it is a two-stroke motor so let's start with a slightly bigger diagram and I'll show you the basic sort of layout of a Detroit Diesel so with a Detroit you have a crankcase with oil into some same as a four-stroke you also have valves in the cylinder head Sam's a four-stroke in this case though you only have exhaust valves not intake valves what you do have though is ports in the side of the combustion chamber like the more traditional petrol two-stroke what you don't do though is use the piston going up to draw air and fuel and oil into the crankcase because of that there's no way for fresh air to get into a Detroit Diesel without a blower on it's a root supercharger same design that you see a lot of you know drag racing cars to give you boosts in this case it's referred to as a blower though because all it's doing is pushing fresh air into the combustion chamber you're not ending up with compressed air it's not actually supercharged as such you're getting kind of an ambient pressure there's a volumetric efficiency thing where piston can't actually draw one atmosphere in because it's coming in through a small space you end up with a you know a slight vacuum it co less than one atmosphere but you know that's just detail okay so we have a crankcase with oil in it we have normal slipper bearings like you have on a four-stroke we have exhaust valves like you have on a four-stroke but we also have a blower drawing air in and ports on the side of the piston like we have on a two-stroke okay so this doesn't get all to theoretical we'll head out in the sunlight and I'll show you so this is the cylinder liner from a Detroit Diesel these line is going to the block and they form the combustion chamber then on the side here you can see these ports that's what they look like they're all round the cylinder liner and that's where the air goes into the combustion chamber this is a piston from up from a four seven month so what you can see is you can be compressing air as the piston comes up to the top of the liner are we filming here dude then once the piston gets all the way down to the bottom of a stroke pressurized air from the blower can come in these ports when fresh air gets pushed in through these ports from the blower not only does it put fresh air in for the next power stroke it also then pushes the old exhaust out through the exhaust valves then it comes up and as soon as that ports blocked off compression begins so on a Detroit you'll hear about the air box the air box drains the air box covers all this kind of stuff and that's what these are in this space around the cylinder block inside the cylinder block so your air box covers here if you take that cover off you can look into the air box you can look through the ports on that cylinder liner and depending on wearing a stroke the piston ears you can see the piston and the condition of the Rings and scoring and that kind of stuff okay this is the phase in Detroit land they still call the scavenging phase which is a very common two-stroke term it works slightly differently normally with a two-stroke the design of the exhaust is critical for getting those exhaust gases to come out in this case we've got the blower pushing them so what we need is all this pressurized air so the blower pressurizes the air box then when the piston is down the ports open fresh air comes in that's the moment then when the exhaust valves open so fresh air comes in pushes the exhaust out so you're replaced carbon dioxide with fresh air so what you can see is with our blower these engines can't operate they've got no way of drawing fresh air or really even getting the exhaust out without a blower so when someone tells you I've got a supercharged diesel it's not it's either until you add a turbo as well which case you can pressurize the combustion chambers but in this case the blower is purely a part of making it work at all all right once that's finished and the pistons come back up past the the port in the side of the cylinder wall the exhaust valve closed and you start compressing that fresh air once it's compressed at the top of the cycle the fuel injector injects diesel into the pressurized air and the heat of that combust sit and you begin your power stroke on the way down now you can see that the Detroit Diesel seems like a bit of a hybrid we've got some oil that's picked up by an oil pump put in two pressurized galleries with slipper bearings so the whole lubrication system works identical to a four-stroke motor we have valves like a four-stroke motor but they're all exhaust valves is no intake valves all the air comes in through those ports on the side of the cylinder wall and it's all pushed in there by diploma now one other area where Detroit's are slightly different to many other diesel engines is that you don't have a high-pressure diesel pump that's timed off the engine that then pushes high-pressure diesel to an injector the pressure of the diesel cracks the injector and the injector burns in this case each injector called a unit injector has low pressure fuel just flowing up to it and returning from a fuel pump on this side of the blower and then the injectors actuated by the camshaft that's where it gets its timing from and the camshaft actually pushes the piston and the injector down to pressurize the fuel and put it in there's a few advantage to this one of them could fail and you can still get fuel to the others if high pressure injector pump fails on a regular diesel you lose fuel to all of them and what it also means the reason you have to bleed a diesel is that if you get air in the line the high-pressure injector pump pushes diesel in but the air in front of it compresses liquids don't really compress but there does and because it compresses it never reaches a high enough pressure to open the injector to allow that air out so all you end up doing each cycle is compressing the air pushing of air compressing the pushing it back it's not till you bleed the injector by opening it that the air comes out and you eventually get fresh fuel you lock it off and you're good to go in that sense the Detroit Diesel is a common rail diesel in that you have this common rail going to all the injectors with a modern common rail though that is a really high pressure line and that you have a sort of piezo mechanism for opening the injector and letting that high-pressure diesel out in the case of the Detroit it's a low pressure common-rail which the injector then pushes up to a higher pressure by the plunger inside it so to sum it up for strict lubrication exhaust valves the fuel does nothing to lubricate the system like it does in a petrol two-stroke lawyer you add oil to the fuel the diesel lubricates can cause the injectors if you run them dry with no fuel you will damage them but the diesel doesn't have any role in lubricating the crankshaft likely fuel and oil mixture does in a petrol two-stroke with the Detroit Diesel we have pressurized air in the air box that's been pressurized by the blower we haven't drawn air into the crankcase by the piece of traveling up and we haven't compressed that air in the crankcase by the piston traveling down there's no transfer port like there is with a petrol true stroke essence so I'm not a Detroit mechanic I'm not an expert on these engines by any stretch of the imagination but this is their general operating principle a lot of people understand a basic four-stroke and understand a more traditional petrol two-stroke but you know it's not that common to understand how these work which is why the question is quite common and hopefully it explains it oh well take care I've now taken the blower that I got from Andrew and given it to pizza who is Adrienne's dad really love a guy I met him very briefly but sort of god I'd like to go and meet again never be it was really nice and so Adrian will be picking it up from his dad's house and next week we'll be basically building the best blower he can from the two we've got and getting it back on the engine all right so hopefully next week we'll be looking at that and I'll catch it in [Music] you

Channel: Dangar Marine

Views: 78,656

Rating: 4.9501371 out of 5

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Id: Xa2x5vrie-o

Length: 14min 26sec (866 seconds)

Published: Sat Dec 14 2019