Why are two stroke direct injection engines uncommon?

Question

Traditionally, there have been two kinds of engines: two stroke and four stroke, and diesels exclusively were four stroke (apart from maybe very large marine diesels).

The two stroke engine has some advantages:

• Simplicity and low cost
• High power to weight ratio due to having power stroke twice as often

As well as some disadvantages:

• Fuel inefficiency due to some fuel being blown right through the cylinder into exhaust
• Smelly exhaust due to high hydrocarbon emissions, due to both lubricating oil mixed in the fuel and due to fuel and lubricating oil being blown right through into exhaust
• Poor lubrication leading to short service life
• Necessity of mixing lubricating oil into the fuel, with immediate engine damage if this is forgotten
• Necessity of having freewheel on vehicles driving downhill and engine braking, to ensure adequate lubrication

However, with direct injection and using the crankcase as an oil pan all of the disadvantages of two stroke vanish:

• Fuel no longer blown right through into exhaust
• Exhaust doesn't smell anymore because unburnt fuel and oil are not part of it
• Lubrication would be as good as in four strokes leading to long service life
• Lubricating oil no longer needs to be mixed into the fuel
• Freewheel no longer necessary due to having lubrication system identical to four strokes

What I'm wondering is why we don't see any of these direct injection two stroke engines in cars. Maybe some high-tech 50cc moped could use one (probably due to 50cc restriction with anything larger being a motorcycle).

About the only disadvantages of a direct injection two stroke engines are:

• Fuel injection needs to happen at very high pressure because fuel needs to be injected after the exhaust port is closed and the pressures are very high then. Yet modern diesels manage to do this.
• Maybe limited fuel evaporation speed could limit RPM if the engine is a gasoline engine -- usually direct injection gasoline engines inject fuel on the intake stroke, not on the compression stroke
• Supercharger is necessary for scavenging the cylinder, but it doesn't increase air pressure in the cylinder like superchargers usually do. This adds weight and cost.
• Turbochargers aren't compatible with a two stroke direct injection engine since the intake and exhaust ports are simultaneously open so it would just blow air right through the engine

Actually, a two stroke direct injection engine could maybe be more fuel efficient than an equivalent four stroke because it has a high power to weight ratio so a smaller engine can be used.

I'm not suggesting that chainsaws should switch to direct injection since the injection system and supercharger add a lot of weight and cost. Yet I think such a two-stroke direct injection engine could be very ideal in cars. However, we don't see such engines in cars, we see them only in large marine diesels. Why? Is there some fatal drawback that I'm not just seeing?

Answer 1

There are many reasons why they are not in cars, but i will write some.

1. Emissions, in Europe we have EURO 6 emission standard, which manifacturers have big headache to deal with. In short, EURO 6 car engine is controlled by exhaust system. On Two stroke, very hard to achieve.

2. The RPM range. Cars drive daily with totally variating engine speeds, you want maximum torque in all over the RPM range. (N/A BMW engines are doing great at this). ---Another reason, why these are only used in locomotive or marine engines, is that they are constantly working on fixed engine speed. At which speed they are DESIGNED for.

3. Fuel consumption. Gasoline: Even if its direct injection, you cant run too lean and not too rich. Hard to achieve correct AFR with traditional two stroke design. Diesel: Eventho marine engines are two stroke, because like you said, they have basically twice the power. You have hard time having precise fuel quantity for different engine speeds and loads, even if two stroke has weigh to power advantage, this is barely noticable on small applications like cars. Oh.. and turbocharger which is the reason Diesel became popular in cars.

There are alot more reasons, but these should give you a little more horizon.

Also, all these Marine engines also have exhaust valve, so even if you would like to have two stroke in cars, you cant make viable two stroke without using somekind of valve system. Traditional (motorcycle) gasoline two strokes only suck air because crankcase has vacuum when piston is travelling up. For this kind of application you can't ever use forced induction.

Wanting to use direct injection and forced induction? The marine engine is what you get. Want them in cars? Well, get the same fuel consumption as four stroke engines and meet the emissions.