What makes two-stroke engines less fuel-efficient than four-strokes?

Question

It is a commonly-held belief that two-stroke engines are less fuel-efficient than four-stroke engines, and some example BSFC figures appear to confirm this as well.

But what is it that causes two-strokes to be less fuel-efficient?

I used to believe that it is due to the fact that the intake stroke occurs once per two revolutions in a four-stroke engine, in contrast to the once-per-revolution of a two-stroke engine, but am not so sure now.

This Evinrude article suggests that the difference in fuel efficient is down to the method of fuel delivery, so the differences in fuel economy is due to an unfair comparison of older two-strokes with newer four-strokes.

So what factors would explain the differences in fuel economy between the two types of engines?

Answer 1

Just so we're on the same page as to how two strokes work, here's a pic. I had to look it up because I had the wrong picture in my head.

In looking at how the cycle actually works, the power stroke goes off creating the combustion products and power. As the downstroke begins the pressure in the cylinder is high allowing the exhaust gases to escape and forcing the intake reed valve closed. As the upstroke happens the pressure in the cylinder is now low because the escaping exhaust gases cause a small pressure wave of escaping gas that now opens the reed value and draws in new fuel / air mixture.

It seems like some key reasons for the engine being inefficient:

• The cylinders are not scavenged of exhaust gases by the piston forcing them out, they simply escape because the outside air pressure is lower than the cylinder pressure after the spark ignites the fuel. This would lead to incomplete expulsion of the exhaust gases. The volume consumed by those leftover gases prevent more air / fuel mix from being ingested.
• As the upstroke happens, for some part of the travel the air / fuel mix is also being expelled. Thus wasting fuel as it's expelled.
  
  Maybe these problems are solved in larger two strokes but the small ones drive things like weed cutters, snow blowers, lawn mowers, etc, small engines for limited applications. Not driving cross country. For these small engines parts count and cost are much more important, so they work really well for those applications.

Answer 2

The efficiency of any internal combustion engine is directly related to its Carnot efficiency, where efficiency equals the inlet air temperature minus the exhaust temperature divided by the inlet temperature. This is directly influenced by the expansion ratio of the gases. A diesel engine has an expansion ratio approaching 30:1 whereas a gasoline engine rarely can exceed 13:1 due to considerations of detonation with average fuel octane ratings. In a conventional two-stroke engine, the exhaust must open very early in the power stroke to enable the cylinder pressure to fall well below that of the incoming charge, in order to avoid spent gases entering the transfer ports and mixing with the fresh charge. The higher the operating rpm, the greater the necessary exhaust lead (called "blowdown"). Generally, the expansion ratio is equal to the expansion ratio in piston ported two-stroke engines. In four-stroke engines the exhaust ports are generally opened just prior to bottom dead centre piston position, thus giving a maximum expansion ratio. In a two-stroke the exhaust may open as much as 90 degrees before bottom dead centre, thus wasting 50% of the power stroke and drastically reducing efficiency at the expense of high power output at higher rpm.

Answer 3

I have to both agree and disagree with your statements in the question and the article.

The higher fuel consumption of a two stroke engine is mostly due to the fact that it has a power stroke per revolution of the crankshaft.

I however have to disagree with the article stating that the fuel delivery plays a major role in fuel efficiency of older 2-Stroke engines.

I support my statements by taking the example of fuel efficiency difference between a carbureted 2 stroke & a carb 4 stroke engine. Not even considering EFI when both are carbs the 4 stroke still out performs the 2 stroke by significant amount.

• A Yamaha 125cc 2stroke engine gives around 70mpg
• A honda 125cc 4stroke engine gives around 153mpg

Now obviously EFI may it be Direct injection or port injection will improve the efficiency and exhaust of any engine regardless of it being a 2 stroke or 4 stroke.

The E-TEC technology shown in the video is mearly a GDI on a two stroke engine, it will increase the efficiency but will it be equal to a GDI 4 stroke engine of the same capacity? I highly doubt it , for example

• The EFI version of the above 125 cc honda engine gives around 166mpg

Meaning if the 2 stroke suzuki engine with GDI can produce more than double the F.E then I agree with the concept but with my knowledge of how GDI works I am not certain of it.

Note : The engines are from Yamaha RX135 , Honda Stunner and Honda stunner PGM-FI and these are real world figures.

Answer 4

Very much depends on the particular 2 stroke and 4 stroke engines. But a major advantage of a 2 stroke is that they can be produced incredibly simply and so cheaply. An engine with 3 moving components (crankshaft, con rod and piston) is probably not tweaked for fuel consumption.

The biggest issue is probably that the exhaust port is open while the intake mixture is being taken in. Hence a potentially large amount of unburnt fuel disappears straight down the exhaust having performed no useful function (beyond maybe cooling the engine a bit).

Further fuel atomisation is likely not helped by feeding the intake mixture through the crankcases and ports, giving the fuel more chance to form larger droplets.

On a performance 2 stroke the exhaust will be designed to draw mixture through the engine, both burnt exhaust gas out and fresh mixture in. Likely more fresh mixture will be drawn through to the exhaust , before pressure waves push this mixture back in to the engine. This works well for getting extra fuel (and hence power), but is not so good for economy. Further it only works at certain rpm ranges.

Some of these issues can be fixed with direct fuel injection (and there have been production 2 stroke motorcycles with direct fuel injection engines, and Ford produced a batch of Fiestas in the 1990s with 2 stroke engines for evaluation purposes). But direct fuel injection is an expensive and complex addition to a simple engine. With such a system air can be taken in to the engine with the fuel only injected once the exhaust port is closed.

The 2 stroke engine does have a major advantage over the conventional 4 stroke engine. Without needing to contain valves the combustion chamber can be far more easily shaped to suit the purposes of that particular engine.

Answer 5

Take a look at how a 4 stroke engine works.

a) Downstroke - sucks mix into the engine

b) Upstroke - compresses gasses

c) Fire

d) Downstroke - engine does Work

e) Upstroke - used gasses are expelled

Now look at the 2stroke

a) Fire

b) Downstroke Engine does work (high pressure in cylinder) Compresses mix in crankcase

c) Upstroke - Engine must both exhaust gasses and get new mix in - sucks new mix into crankcase

There is thus always going to be a mix of exhaust and unburned gasses in a two stroke engine. There used also to be a time when, to increase power, the two stroke transfer of mix from lower crankcase overlapped the exhaust port being opened. This resulted in unburned fuel going straight through the engine.

Modern design reduces but cannot entirely remove these efficiencies which appear still to be more taxing than the 4 stroke method of turning the engine twice to get one work stroke.

Answer 6

Its very simple. In a 2-stroke the fuel is also the lubricant and coolant and mixing oil with gasoline increases the energy content of the fuel while decreasing the octane so 2-strokes have to run fixed ignition timing and super rich fuel-air mixtures made even richer by the high-energy, low-octane oil mixed in. Air-cooling makes them even more sensitive to timing, cylinder temps and other variables their fixed timing and total loss lubrication and cooling systems can't compensate for. And of course they have huge, built-in vacuum leaks that occur at the worst possible time for volumetric efficiency and valve timing is also fixed while even with a mechanical lifter cam a four-stroke engine's valve timing advances as engine speed increases. For any given amount of valve lash the faster the engine runs the more quickly that lash is taken up and the sooner valve events occur.

2-strokes were competitive against low-compression, low-speed flathead engines for seasonal use only until metallurgical and manufacturing advances made it possible to build cheap 4-stroke OHV engines with electronic ignition and fuel injection so ignition and fuel tuning became automatic and optimal for the more automotive and seasonal applications like snowmobiles, ATVs, outboard motors, outdoor power equipment like string trimmers and leaf blowers and other consumer products. Electronic ignition system improvements and dedicated, application-specific carburetion and professional grade performance expectations and price points are barely keeping 2-strokes competitive in industrial/commercial machines like portable cutting tools. Cut-off saws, chainsaws, etc.