Why are 2 stroke diesel engine not commonly used?

Question

The 2 stroke petrol engines have the disadvantage of loss of fuel/air mixture during suction into the exhaust but 2 stroke diesel engine doesn't have such a disadvantage as it is air that is sucked during suction.so why can't we use 2 stroke diesel engine with efficiency than its counterpart of petrol engines?

Answer 1

Two stroke diesel engines require a blower to provide scavenging air.

After the power stroke reaches bottom dead center exhaust valves open and either intake valves or intake ports open.  Pressurized air from a blower or turbo charger then enters the cylinder to purge the exhaust gases and load a fresh charge of air for the compression stroke.    The fresh air is compressed and diesel fuel injected near top dead center and the the power stroke occurrs for the next cycle where the process repeats it self.

The 2 stroke produces a power stroke every revolution compared to  every other revolution for a 4 stroke. The apparent production of twice as much power the 2 stroke produces vs 4 stroke is offset by the energy required to drive a blower, cost of the blower and additional maintanence, increased weight and size of the blower.

2 cycle diesel engines with Roots blower have been known to overspeed and go into a run away condition when an oil seal in the blower fails.  Lubricating oil leaking into teh blower is fed into the the engine where it acts like additional fuel.   Intake dampers are fitted between the blower and engine which are designe to cut ouff the air to the engine should a run away condition exist.

Imagine driving a 2 cycle engine that stops responding to the throttle and continues to run after you turn the key to switch it off..

Answer 2

There are market segments where a 2-stroke diesel engine is very common.

In the railroad market, the most common diesel engine is a 2-stroke, and is competitive with all the 4-stroke engines of that size range.

What's more, the 2-stroke engine is lighter (and simpler) than its 4-stroke cousins, meaning less maintenance.

Yes, the engine does require a blower for scavenging air, but all diesels in the 2000+ horsepower range use them (or turbochargers)anyway, for increased hp-to-weight ratio.

By the way, the runaway condition earlier mentioned is not specific to two-stroke diesels, but is a particular risk that all roots blown or turbocharged diesel engines face.