3
$\begingroup$

Of course, most 2-stroke engines require oil to be mixed with the fuel (I appreciate there are 2-stroke engines that don't require this). Therefore, it makes sense to me that oil will be burnt during the combustion process, given it is mixed with the fuel.

However, what doesn't make sense to me is why 4-stroke engines don't burn just as much oil. In both engines, my understanding is that the oil has to be used within the cylinders to keep them from seizing, so surely a 4-stroke engine would also be burning oil too, with the oil just being added at a different stage?

I would like to understand why 2-stroke engines burn more oil than 4-stroke engines?

Improve this question
$\endgroup$
1
  • $\begingroup$ An equivalent question might be to note that both engines need oil for lubrication but get it by different methods and then ask why the 2-stroke needs to have so much more oil added to the fuel than would normally be consumed by a 4-stroke. I.e. what would go wrong if only half the recommended amount of oil were mixed with the fuel? $\endgroup$
    – Ray Butterworth
    Commented Apr 13, 2019 at 12:50

2 Answers 2

Reset to default
2
$\begingroup$

In both engines, my understanding is that the oil has to be used within the cylinders to keep them from seizing, so surely a 4-stroke engine would also be burning oil too, with the oil just being added at a different stage?

Oil is not added because piston rings separate it from fuel. Rings keep the oil at one side of the piston (the crankcase side) and the burning fuel-air mixture at the piston head side. Ideally, the oil is not supposed to get to the head side at all: no mixing and no burning.

Of course, some is oil is left on the inner cylinder wall in the form of a film, some of it get washed away by fuel and burned by the combusting mix. But this amount is much, much smaller than the amount of oil that has to be mixed into fuel in cheap 2-stroke engines. While the mixed-in oil passes through the crankcase just once yet it still has to provide similar level of lubrication as the oil that's (almost) permanently contained in the oil pan of a 4-stroke engine (or a proper2-stroke engine with a dedicated scavenging blower, as you've mentioned).

Basically: 4-stroke keeps reusing oil and loses only as much oil as slips past piston rings. 2-stroke uses oil once and throws it overboard. That's why so much more oil is necessary when it's mixed into fuel.

Improve this answer
$\endgroup$
0
2
$\begingroup$

Simply because many small 2 strokes use the oil / fuel mix as the lubrication system as the lubrication occurs as the mix passes through the crankcase on its way to the combustion chamber.

A four stroke, and some 2 strokes, have a dedicated oil lubrication system with an oil reservoir, often in the sump, so the oil is not consumed with the fuel.

4 strokes do have a small amount of oil left on the bore as the piston descends because the scraper ring is not perfect while it does remove most of the oil film... But this is a really tiny amount.

Improve this answer
$\endgroup$
2
  • $\begingroup$ Thanks for your answer. One question I do have is how the piston's movement within the cylinder is lubricated in a 4-stroke engine without oil being on the cylinder walls while combustion occurs? $\endgroup$
    – PhysicsGuy123
    Commented Apr 13, 2019 at 14:14
  • $\begingroup$ Only the rings are in contact with the walls.. and as mentioned there is an oil film... $\endgroup$
    – Solar Mike
    Commented Apr 13, 2019 at 15:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.