Why isn't more work being done on developing advanced 2-stroke internal combustion engines?

Question

Given that a 2-stroke configuration increased power density two-fold over a 4-stroke configuration, why does it appear that efforts to develop a more fuel efficient 2-stroke engine are minimal, at best? 2-stroke engines utilizing valves rather than ports have been developed, but have never been commercialized. I'm trying to find out what are the primary obstacles/objections to developing such an engine - either gasoline or diesel.

Answer 1

I am not an expert. But I have read that, despite the power density advantage, they have some serious inherent disadvantages. One huge one is emissions control - because the incoming charge has to scavenge the exhaust, it is exceedingly difficult to make sure that the cylinder is perfectly purged of exhaust every time, without either retaining too much exhaust or allowing unburned mixture out into the exhaust manifold, over the full range of speed and load. The 4-stroke design is much better this way. Not incidentally, this issue affects fuel economy as well as emissions.

Another big problem is that in the traditional 2-stroke design you need to include oil in the fuel. This is a huge killer for emissions. To avoid it you have to have a sealed crankcase and some form of externally powered induction, and then all of a sudden the simplicity, size, cost and weight advantages are compromised.

Answer 2

Power density is not important in most applications of an internal combustion engine, but fuel efficiency usually is. And a 2-stroke engine will never be as fuel efficient as a 4-stroke engine.

The diesel engines on a cruise ship are not a significant portion of the total weight of the ship. And ships don't care about weight anyway.

A device has to get pretty small, like a motor scooter or a weed-wacker, for the weight of an engine to be important.

If power density is really critical, then a jet turbine is used, like in a passenger jet or the space shuttle main engines.

Answer 3

One main problem is heat dissipation. You have to get rid of the heat you create from the combustion. Current 2-strokes mostly vent the heat out with the exhaust gasses, leading to further reactions with atmospheric gasses and increased pollution and noise. You can see this in the applications where 2 strokes are being used.

These problems are currently under review and we will probably see regulations out of congress or the EPA soon.

The other way heat leaves these engines is by way of the walls of the piston. But 2 strokes have a larger combustion chamber in ratio to the piston walls, so they tend to heat up faster. The slower 4 stroke actually has two advantages here, both a lower working speed per cylinder and a smaller combustion volume per cylinder.

You question is the same trade off you get if you try to make a gun fire faster. Sure, you get more down range and quicker, but the cost is in the damage done to the gun.

Answer 4

One big problem with two-stroke engines is that they need oil in their fuel to lubricate the piston rings. That means they make considerable smoke, a very undesireable thing when the world is trying to reduce pollution.

The thermodynamic efficiency of the two-stroke is also slightly less than that of the four-stroke.

Considerable R&D has raised the power density of the four-stroke probably beyond that of many four-strokes.

Answer 5

First, let me say that a lot of work has gone into the automotive engine in that area. There are numerous engines converted directly to 2 stroke by Chrysler in Wisconsin. Chrysler Marine has a whole grouping of these engines in significant horsepower ranges that have been on the market for a number of years. I suggest you go to the nearest marine yard and see just how this is done.

The problems range from a huge number of stainless steel reed valves onto smoky engines that will not meet standars of emissions. True the HP vs eff. is imporved but one is left with an engine that has low torque at slow speeds. This has not hampered the usage in marine engines, as the higher torque requirements are at high propeller speeds and low torque is physically present at slow speeds of the impeller.

Answer 6

The Folk's at Harley Davidson have done a pretty good job
on their V-Twin engine with the screeming eagle carb set-up.
Sorry U won't find any in cars, not enough engine.