4-stroke vs. 2-stroke lubrication

What’s the Straight Dope on why 4-stroke engines can have external (as in not in the fuel) sources of lubrication, whereas oil must be mixed into the fuel of a 2-stroke engine?

I understand the fundamental differences between the two (number of strokes in between a powered one, etc.) but I don’t really see how this would translate to the lubrication system.

Thanks for any input.

Crankcase compression.

A four-stroke engine never has the inlet charge enter the crankcase.

The four strokes “suck, squish, bang, blow” allow the entire gas exchange process to be done above the piston. A gross oversimplification of the process follows. The intake valve opens, the piston comes down the cylinder, draws in the air. The intake valve closes, the air and fuel are compressed by the rising piston. The spark fires, the piston goes down the cylinder. The exhaust valve opens, the piston comes up, and the exhaust is pushed out.

A two-stroke is more complex. Here’s what I wrote for a go-kart website:

The two-stroke engine is used in karting from Comer 50 Kid Karts to 250 SuperKarts. It is an internal-combustion engine with the ports in the side of the cylinder determining when intake charge and exhaust can move in and out of the cylinder. This therefore uses the piston as the valvetrain, eliminating the complex contraptions that control timing in four-stroke engines.

Perhaps the easiest way to understand how this engine works is to track a small volume of air as it travels along. We’ll assume this volume of air is lucky enough to get sucked into the airbox.

First, it travels through the air filter, if there is one. The filter serves two purposes, to remove dirt from the air and to prevent pressure waves from traveling out of the airbox creating noise. Once through the filter, it goes into the carburetor. In the carburetor, fuel and oil is added to the air (see the article on how carburetors work to see how this happens). Not much of either is added. By mass or weight, 12 to 15 times more air is used than fuel, and fuel is mixed between 13:1 and 50:1 with the oil. Of course, quantity has nothing to do with importance - would you drink a gallon of water with three drops of hydrogen cyanide in it?
Once the air has some fuel and oil in it, it is ready to enter the crankcase. On the up-stroke of the piston, a vacuum will be created in the crankcase, which is otherwise sealed. This will suck the air through the entry port into the crankcase. Air will continue to enter the crankcase until the pressure has equalized.
The piston will then come down the crankcase. The first thing this does is to raise the pressure in the crankcase, preventing more air from entering. It then seals off the entry port to the crankcase, preventing air from being forced back into the carburetor. Finally, it opens up the transfer ports in the cylinder.

Now comes the exciting part. As the piston continues to travel down the cylinder the pressure in the crankcase will rise. The entry port is closed. The walls of the crankcase are sealed. There’s only one place for the air to go - up the transfer duct. The transfer duct runs outside the cylinder wall, going from below the piston to above it. Once the piston goes past the top of the transfer port in the cylinder, the air will enter the cylinder.
Air will continue to enter the cylinder as the piston drops and then begins to rise. Once the piston goes up far enough in the cylinder, the transfer ports will be sealed off and the cylinder will now be sealed up. The piston will continue to rise in the cylinder. Since the volume continues to decrease but the mass contained doesn’t, the pressure and temperature of the air will increase considerably.

Now the fuel becomes important. Shortly before the piston reaches its highest point, the spark plug gap will be bridged by a 5000+ volt spark. When this happens, the fuel begins to burn in the air. This supplies a VERY large amount of heat. This will cause a proportionally large pressure rise in the cylinder. This forces the piston down the cylinder very quickly. The fuel will continue to burn as the piston drops. Ideally, the piston will reach the top of the exhaust port just as the last of the fuel burns.
The piston will eventually open up the exhaust port. Since the cylinder pressure is much higher than the outside pressure, the burned fuel and air will exit out the exhaust port and into the exhaust. If you forgot to check your exhaust springs, it will go directly into the atmosphere. Otherwise, it will travel into the exhaust pipe. The purpose of the exhaust pipe is to use the pulse waves from the exhaust to provide reduced pressure to draw the exhaust on the NEXT stroke of the engine out of the cylinders.
When the exhaust port opens the exhaust will begin traveling down the pipe. As this happens, a pressure wave (sometimes known as a shockwave) will be created. The wave will travel at the speed of sound down the pipe and bounce off the other end. As long as the wave is moving away from the cylinder, the pressure will be less than the outside pressure - the wave will draw the exhaust gas along with it. When it is moving back towards it the pressure will be higher. The exhaust gas, when it reaches the end of the pipe, will leave via a hole and go into the atmosphere.

Ideally the wave from the last pulse will reach the port right before the piston opens it - that way both of the waves will help move the gas out of the cylinder. The more effectively exhaust is moved out of the cylinder, the more fresh air can be moved into the cylinder. As long as you can burn all of the air/fuel mixture you bring in, the more fresh air you move into the combustion chamber, the more power you will have.

Several means other than the simple piston port are used to control the flow into the cylinder. These provide significantly increased power. A reed valve is a simple one-way valve placed between the carburetor and crankcase. It prevents air from moving out of the crankcase and back into the carb. It consists of a flexible reed covering a hole. When the pressure is lower in the crankcase, the reed bends out of the way and the hole opens This allows a larger intake port and more aggressive port timing. A rotary valve allows precise control of intake timing. It consists of a disc, turned by the crankshaft, that opens and closes the port. Reed valves are used in Rotax, TaG, KF, ICA, and shifter karts. Rotary valves are used by Formula C karts.

Two-stroke engines have many advantages over four-strokes for karting. The first few have to do with their mechanical simplicity. They have much less inertia - the only moving parts are the piston, ring, connecting rod, and crankshaft. A two-stroke is much smaller and lighter than an equivalent four-stroke. They can also be rebuilt very quickly and sometimes even inexpensively as very few parts have to be replaced.

The lack of a valvetrain provides several more advantages. Since there is no need to move air through the head, the combustion chamber can be close to the ideal shape for efficient combustion. In addition, there is no power lost to the considerable mechanical drag of the valvetrain. This allows for excellent energy efficiency - a HPV or Rotax-powered kart can get 40 MPG in sprint racing, significantly better than any four-stroke. Finally, the power characteristics of the engine can be changed with stationary components - the powerband can be changed by changing the length of the exhaust pipe.

Against these come several problems. Since the only oil supply is mixed in with the gasoline, lubrication is mostly by luck and rebuild intervals range from 3 to 50 hours. The oil is also burned in the combustion chamber, and that means that emissions are higher. Air and fuel can also leave the cylinder via the exhaust port before combustion. Finally, pressure waves are also sound waves - two-stroke engines are loud. Oiling on many advanced engines is by direct injection feed to the bearings. It is still a “total loss” system but the consumption is far lower. .

Most two-stroke engines use the crankcase as part of the induction system and this is why they require the oil-gas mix. There are large two-stroke diesel engines out there that don’t use the crankcase as part of the induction system, and they don’t use the fuel and oil mix. Check it out here- http://auto.howstuffworks.com/diesel-two-stroke1.htm

it’s not inherent in the 2-stroke cycle that lubricant must be mixed with fuel. in the case of small engines, it was just way more convenient and cost-effective to do so. as has already been posted, large diesel engines are frequently 2-stroke, and even some of the smaller truck engines in the past (e.g. Detroit Diesel) were 2-stroke. On the gas engine side, even Chrysler was planning to release the Neon in 1995 with a small, external breathing 2-stroke engine which did not mix the lubricant with the fuel.

Thanks for the responses all.

I suspected that it had something to do with the different location of the inlet (at least on small engines) but I didn’t realize that this wasn’t a fundamental part of the design for two-strokes.

Makes you wonder why they don’t have more two-strokes on econoboxes (ala jz78817’s Neon). The proportional weight savings on a CRX-sized car could be pretty big.

(That’s quite an article, vactrac! Maybe there’s an SDSAB membership in your future!)

2 strokes are not as efficient as 4 strokes. In an airplane, or a chainsaw, the much lower power density makes them valuable, but in a car they’ll never compete with 4 strokes, for gasoline engines. For diesel and similar derivatives, 2 strokes may be competitive.

Ask the locomotive guys and shipbuilders about it - the two-strokes are significantly more efficient. Of course, those are valved “uniflow” engines.

The critical factor for a car- or motorcycle-size engine is direct injection. Without it, around 30% of the fuel goes straight out the tailpipe unburned or partially burned. With it, there are major efficiency advantages, up to 15% gains against the best four-strokes. The pumping losses (induction/exhaust) are much, much smaller, and you can have whatever head design you’d like because you have no valves. With no valvetrain you also lose that source of mechanical drag.

Power density barely matters in modern, 3700 lb midsize cars. If you were to design an engine for a 2000 lb car, however, it would be foolish to rule out a 100+ lb weight saving on a 100 hp engine. The main reasons the automakers haven’t gone for it is customer acceptance of an exceptionally “peaky” engine, the difficulty of guaranteeing good lubrication for the lifespan of the engine, and some concerns over particulate emissions.

2 Strokes can be more efficient and have greater power density than 4-strokes.

At present a company licences its 2 stroke technology out to Aprilia.In this engine the lubricant and fuel are seperate, and the crankcase is not used as the pre compression stage, instead the fuel air mic is compressed in an external pump driven by the crankshaft, this is then injected and ignited. Much if the inefficiency in 2-strokes has come from the way that the inlet and outlet ports operate, but in this engine the injection system operates to ensure that ignition can only occur when both ports are fully closed and burn is complete by the time they open, no matter what the revs.



There is potential to scale up this engine, Yamaha have been working on their own version, things have gone pretty quiet over the last few years so I presume that there are issues that have not been fully resolved.

Anyone else wander in here expecting to see NSFW?

even in externally scavenged, direct-injected engines, emissions have been problematic.


DI two-strokes are still fairly popular in marine outboard engines which don’t have the same emissions restrictions as cars:

http://www.youtube.com/watch?v=aTMxCLUIZ7M (Mercury two-stroke V6)

There used to be a lot of two-stroke small diesels in trucks- especially popular in buses and fire engines since Detroit Diesel V6s and V8s could package a lot more compactly than the typical big inline 6 that others offered:


again, they stopped selling these for on-road applications in the late '80s because of emissions. They were extremely dirty (and rather loud, too.)

Basically, as the above descriptions point out, intake and exhaust ports are open at the same time, and the pressure of the intake blows the exhaust out. There is some inevitable mixing and exhaust of unburnt gas - or else you limit the flow so you don’t fully effectively use the volume of the cylinder and the fuel-air mix would be uneven. So emissions would be a problem even if the engine was not also deliberately burning oil. Add to that the tail end of the explosin in the cylinder exhausts into the muffler (i hope there is one) for noise problems. In a 4-stroke, the explosion finishes and the cylinder pushes out the exhaust on the next upstroke.

Glad to hear you enjoyed yourself. Oh, wait, is that your signature?

finally i got to picture it right. the 2S really has fuel/oil transfer between crankcase and combustion whereas the 4S all happens above-piston (intake-compression-combustion-exhaust.)

my question is this: for a given cylinder displacement and fuel type, should there be a serious difference in performance (HP?) i’m asking this because of our city government’s policy of registering only new 4S bikes, forcing older 2S ones to other regions.

again, this is only true for carbureted, crankcase-scavenged 2-strokes. There are direct-injected crankcase-scavenged 2-strokes that do not run the fuel through the crankcase, and there are external-breathing (blower-scavenged) 2-strokes which keep the crankcase completely separate from the cylinder.



small two-strokes can make a lot of power for their size, but require a tuned expansion exhaust pipe to do so, and only make real power over a very narrow RPM range. once you’re “off the pipe” the engine falls on its face.

well, it’s clear the city gov’ts move is with regard to air pollution. i’m just wondering what other advantages a 4S can offer the public aside from lower fumes emmission.

Noise; 4-strokes tend to be quieter. Efficiency; 4-strokes tend to have more effective aspiration since they have ~180° of crank rotation to clear the cylinder of burned gases and another ~180° to pull in a fresh charge while two-strokes have ~110° of crank rotation to do both jobs at the same time. Broader powerband; as above the reliance on a tuned pipe means a 2S has a fairly narrow powerband.