2 cycle vs 4 cycle

"PartyPooper2012" (PartyPooper2012) 07/23/2020 at 11:53 • Filed to: None

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I guess I am still a bit fuzzy on why 2 cycle engines need oil mixed in with fuel? Why does 4 cycle engines don’t need that.

would 6 cycle engine not need fuel?

Bman76 (hates WS6 hoods, is on his phone and has 4 burners now) > PartyPooper2012 07/23/2020 at 11:57

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Crank case is part of the intake stroke.

(and/or you’re just trolling at this point)

DipodomysDeserti > PartyPooper2012 07/23/2020 at 12:00

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The fuel mixture is pre compressed in the crankcase, so you can’t have oil just sitting in there like in a four stroke. Therefore, in order to get any lubrication, you have to put a bit of oil in the fuel. Wasn’t all that uncommon for old two strokes to seize up while riding. A lot of guys who learned to ride on them will hover over the clutch lever, just in case the rear wheel locks up.

TheTurbochargedSquirrel > PartyPooper2012 07/23/2020 at 12:02

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In a 4 stroke engine the fuel is only present in the combustion chamber. Thus it needs to pump its own oil around to keep the crank and valves lubricated. A 2 stroke engine pulls it’s fuel in to the combustion chamber through the crank case. That means the fuel comes in contact with all moving parts of the engine. By using the fuel delivery as your oil delivery you can eliminate a dedicated oiling system, saving cost and complexity.

Here’s a good animation showing how a 2 stroke engine works:

http://www.animatedengines.com/twostroke.html

Snuze: Needs another Swede > PartyPooper2012 07/23/2020 at 12:05

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6 strokes exist, it’s basically a 2 stroke “engine” that replaces the valves in a 4 stroke engine.  It’s weird and gives only a moderate benefit.  

ClassicDatsunDebate > PartyPooper2012 07/23/2020 at 12:06

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Correct, 6 cycle engines don't require fuel or oil

Brickman > PartyPooper2012 07/23/2020 at 12:13

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2 stroke is mainly used on small boats and lawn equipment, needs oil + gas for lube as there is no oil sump. trimmer can run on straight gas, but not for long as the gas washes away the oil and metal and metal heat up and no more engine .

2 stroke diesels dont need oil, because the engine is large enough for an oil pan and pump,  plus  diesel is an oil. There are only exhaust valves, because the air intake is in the combustion chamber on the bottom dead center pulled in by a roots blower.

A very dirty engine, but quite powerful. Like a DD 8v92 ;D

dogisbadob > TheTurbochargedSquirrel 07/23/2020 at 12:20

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awesome site

PyroHoltz f@h Oppo 261120 > PartyPooper2012 07/23/2020 at 12:48

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6 cycle is essentially an arc reactor, Tony was  the only one who’s been able to make it work.

PartyPooper2012 > TheTurbochargedSquirrel 07/23/2020 at 12:51

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huh! I did not know the fuel goes all over the world in a two stroke. Thanks.

TheTurbochargedSquirrel > PartyPooper2012 07/23/2020 at 12:58

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2 strokes are fascinating things. Using the piston to pump fuel and oil around the engine is a genius idea.

This series is a really cool look at rebuilding a trabant engine and gearbox:

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > PartyPooper2012 07/23/2020 at 13:39

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Now, the Italians made a 2-stroke direct-injection scooter engine. This magical creature had an oil injector to mist the crankcase and keep it lubed, and a DI fuel unit in the head to provide combustion fuel. It consumed less fuel and less oil than a traditional 2-stroke engine while maintaining the mechanical simplicity and power benefits of a power stroke every revolution that really makes a difference in small engine. The crankcase was still u sed to draw in the charge air, but the fuel and oil weren’t mixed until/unless aerosol oil from the crank case made in into the piston through the t ransfer port(s), so they were cleaner running too. They (the early ones at least) were, predictably, an electrical nightmare.

Where small displacement and low power are concerned 2-stroke makes a lot of sense. Mechanical simplicity, lower weight, less parasitic loss, higher power density (also agnostic to orientation, as there is no oil sump) . At the expense of efficiency, emissions, and (very) long term durability.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > DipodomysDeserti 07/23/2020 at 13:45

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Only if you don’t check your oil level and have your oil injector adjusted properly. Most guys I know who ran them hard used a dilute premix on top of the oil injectors in case of Bad Things happening.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Snuze: Needs another Swede 07/23/2020 at 13:48

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As I remember it the added strokes are primary intake and pre-compression. Basically it's a piston-pumped supercharger....

Eury - AFRICA TWIN!!!!!!! > PyroHoltz f@h Oppo 261120 07/23/2020 at 13:57

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In a cave, no less. 

DipodomysDeserti > JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t 07/23/2020 at 14:02

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Or you’re running hard   in 110F weather for hours at a time.

Snuze: Needs another Swede > JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t 07/23/2020 at 14:22

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You are correct. I think it basically did both, it acted as a “valvetrain” but also added precompression. It was weird, and as I recall, while it worked, it never really panned out to make significantly more power or anything.

There was also a 6 stroke that used water/steam in a 2 stroke process - after the normal 4 cycles, instead of the intake opening and starting the process again , a second direct injector squired some water in, the residual heat from the exhaust stroke flashed it to steam and the expanding steam provided a 2nd power stroke, then the steam was exhausted, and then the intake opened and the regular 4 cycle started again.

I think that, averaged over time, the steam strokes were so much less powerful and took so long, that the engine made less power than a regular 4 stroke of equal displacement, though it did make more power per unit of fuel burned. But the added weight and complexity offset that. There were also  problems with the steam wiping lubrication off the cylinder walls, so it exhibited a lot of extra wear.

PartyPooper2012 > JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t 07/23/2020 at 14:44

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sigh... that made me remember another engineering marvel.

My wife’s family friend was selling a dirt bike. 2 cycle, but the oil had its own reservoir and it was not mixed... I don’t think it was italian. Actually, it was 1974 Honda Elsinore MT250

I am so confused.

So basic 2 cycle sucks in fuel through crank case, spits it into combustion chamber. blamo. blow it out the rear. That lubed up the whole thing inside. Now what happens in your italian case and this honda here. who lubes what?

I know some say I am trolling, but I am really confused.

Distraxi's idea of perfection is a Jagroen > JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t 07/23/2020 at 14:49

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Fuel injected 2 strokes were a thing in outboards for a while. My folks’ boat had an Evinrude one. 

Cé hé sin > Brickman 07/23/2020 at 15:18

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That’s not entirely the case. Diesel two strokes don’t need mixture or oil injection, but that’s because they don’t use crankcase scavenging. A petrol two stroke sucks in air/fuel/oil mixture below the piston into the crankcase (crankcase scavenging) and transfers it to the combustion chamber. A diesel two stroke (they’re still used on the really huge ships) has its air blown in by a supercharger or turbo above the piston (blown scavenging) and so has nothing going on below the piston and can use conventional lubrication.

There have been attempts to produce blown scavenged petrol two strokes but I don’t think they’ve ever been succes sful.

Cé hé sin > PartyPooper2012 07/23/2020 at 15:23

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Two strokes don’t have to use mixture. The later two stroke bikes and modern two stroke outboards use oil injection. Either way it’s total loss lubrication because oil in the sump lubrication doesn’t go well with air and fuel being sucked into the crankcase .

Diesel two strokes don’t use total loss lubrication at all. B ecause the air is blown in above the piston rather than being sucked in below it there’s nothing going on in the crankcase and conventional lubrication is used.

Cé hé sin > JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t 07/23/2020 at 15:25

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Ah, but two strokes don’t have to be inefficient. The really huge two strokes in cargo ships are the most efficient internal combustion engines made and are very durable indeed. Emissions are an issue though. .

Cé hé sin > Distraxi's idea of perfection is a Jagroen 07/23/2020 at 15:29

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Still are! Evinrude still make them, although I don’t think the business is long for this world as Bombardier are pulling out.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > PartyPooper2012 07/23/2020 at 16:04

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Oil injection pump. In the case of the Honda, there is a small pump driven by the flywheel that squirts a tiny bloop of oil into the intake manifold just after the carb. This oil gets sucked along with the intake fuel/air charge and slathered onto the engine internals. If working correctly, and properly adjusted, these sorts of systems allow very controlled oil use (drawn from that separate oil tank, which means you don’t have to remember to premix, and can just fill up the gas tank from a regular pump. These sorts of systems always have a low oil warning light, or an easily seen oil gauge so that you don’t run your oil tank dry. The oil tank typically holds as much oil as would be required for 5-10 full tanks of fuel.) They can also adjust oil flow based on RPM, and in some of the fancier systems, even engine load (as measured by engine vacuum) and temperature (in liquid-cooled applications). The Italian DI 2-strokes use the same principle, except the oil is injected fully inside the crankcase and through a high-pressure injector (pump driven by the flywheel again) that mists the oil onto key lubrication points directly.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Cé hé sin 07/23/2020 at 16:12

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True, they operate on a two-stroke combustion cycle, but those big marine engines have pressure-lubricated bearings and pistons (like the Detroit Diesel two-strokes). Though I think some of them (the bigger ones) take it a step further and isolate the crankcase from the intake cycle entirely. They use piston ports, but the intake “runner” is a separate cavity/tube/whatever that is pressurized by a turbo or supercharger, whil e the crankcase remains at atmospheric pressure.  

Distraxi's idea of perfection is a Jagroen > Cé hé sin 07/23/2020 at 16:16

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Good to know - I thought they’d been obsoleted by 4 strokes due to emissions requirements, not that I pay a lot of attention to the marine engines world.

Evinrude are gone, according to the local distributor’s website:

Cé hé sin > JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t 07/23/2020 at 16:59

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Strange beasts, marine engines be they two stroke or four stroke. They have an enormously long bore in relation to their stroke and what I can only describe as a two stage conrod which is intended to prevent any sideways movement of the piston and the consequent wear of the cylinder into an oval shape. Oh, and to go astern in a two stroke you just stop the engine and restart it in the other direction. Not the work of a moment.

kanadanmajava1 > PartyPooper2012 07/23/2020 at 21:34

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There are also 2-stroke diesel engines . Unlike in the spark ignited 2-stroke engines the intake air does not pass through the crankcase. So t hey don’t need lubrication added to the air but they do burn some lubrication oil residues that end up in their intake ports. This causes some exhaust emission problems and it’s one of the reason why they aren’t very common anym ore .

S ome 2-stroke diesel engines do have exhaust valves but some have only ports in the cylinder wall (like the in ta ke ports) . In s ome application a single cylinder is occupied by two opposing pistons. Th is can be done with two separate crankshafts or with a bunch of connecting rods and rocker arms.

But all of them require a blower (nearly always Roots- type ) to keep the air moving to the cylinders and to flush the exhaust gasses out . This is called scavenging.

2-stroke diesel engines have almost gone extinct. The last ones still in production are extremely large marines engines and certain military vehicle engines made in Ukraine.

The upper one is a Detroit Diesel type and the lower is a Commer TS3.

50ford500 > TheTurbochargedSquirrel 07/23/2020 at 21:34

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Simply an amazing find! I really like that guys delivery too. Thanks for sharing!