The 2 Stroke
It may surprise you to learn that the biggest diesel
engines in use operate on the two stroke principle. If you have
experience of the two stroke petrol engine you will know that it
causes more pollution than a four stroke petrol
engine. This is because oil is mixed with the petrol to lubricate
the crankshaft bearings, and a lot of unburnt petrol/oil/air mixture is
discharged to the atmosphere. To learn more about the 2 stroke
petrol engine cycle click here.
The two stroke Diesel engine does not mix fuel or oil with the
combustion air. The crankshaft bearings are lubricated from pressurised
oil in the same way as a four stroke engine.
The two stroke cycle is so called because it takes two strokes of the
piston to complete the processes needed to convert the energy in the fuel
into work. Because the engine is reciprocating, this means that the piston
must move up and down the cylinder, and therefore the crankshaft must
1. The crankshaft is revolving clockwise and the piston is
moving up the cylinder, compressing the charge of air. Because energy is being transferred into the air, its
pressure and temperature increase. By the time the piston is
approaching the top of the cylinder (known as Top Dead Center or TDC)
the pressure is over 100 bar and the temperature over 500°C
2. Just before TDC fuel is injected into the cylinder by
the fuel injector. The fuel is "atomised" into tiny
droplets. Because they are very small these droplets heat up very
quickly and start to burn as the piston passes over TDC. The
expanding gas from the fuel burning in the oxygen forces the piston
down the cylinder, turning the crankshaft. It is during this stroke
that work energy is being put into the engine; during the upward
stroke of the piston, the engine is having to do the work.
3. As the piston moves down the cylinder, the useful
energy from the burning fuel is expended. At about 110° after TDC
the exhaust valve opens and the hot exhaust gas (consisting mostly of nitrogen, carbon dioxide, water vapour
and unused oxygen) begin to leave the cylinder.
4. At about 140º after TDC the piston uncovers a set of
ports known as scavenge ports. Pressurised air enters the cylinder
via these ports and pushes the remaining exhaust gas from the
cylinder in a process known as "scavenging".
piston now goes past Bottom Dead Centre and starts moving up the
cylinder, closing off the scavenge ports. The exhaust valve then
closes and compression begins
The two stroke cycle can also be illustrated on a timing
1 -2 Compression
2 - 3 Fuel Injection
3 - 4 Power
4 - 5 Exhaust Blowdown
5 - 6 Scavenging
6 - 1 Post Scavenging
1. approx 110º BTDC
2. approx 10º BTDC
3. approx 12º ATDC
4. approx 110º ATDC
5. approx 140º ATDC
6. approx 140º BTDC
In the 2 stroke trunk piston engine, the side thrust caused by the angularity of the
connecting rod is transmitted to the liner by the piston skirt or trunk.
It is therefore known as a 2 Stroke Trunk Piston Engine. The skirt of the
piston also acts to seal the scavenge air ports when the engine is at TDC.
This prevents the scavenge air from pressurising the crankcase.
Herein lies the disadvantage of this type of engine: although it has a
low overall height, lubricating oil splashed up from the crankcase to
lubricate the liner can find its way into the scavenge space, causing
fouling and a risk of a scavenge fire. There is also the likelihood of liner and
piston skirt wear, allowing air into the crankcase. This can supply the
required oxygen for a crankcase explosion should a hot spot develop. The crankcase
oil must have additives which can cope with contamination from products of
combustion, and the acids formed during combustion due to the sulphur in
This design of two stroke is generally only used for the smaller lower
powered 2 stroke engines - up to about 5000kW for a V16 engine with a
280mm bore and 320mm stroke.
Detroit diesels manufacture 2 stroke trunk piston engines as do
Wichmann and General Motors. Sulzer used to produce a model which is
sometimes found at sea as did Brons. A cross sectional drawing of their type GV
engine is shown below.