The marine Diesel engine

The marine Diesel

engine

Part one: The two stroke

Authored by: Martin Leduc

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Heat engines are sometimes referred to as being motors. Actually, a motor converts one form of energy into useful work without the intentional production of heat, such as an electric motor. A device which burns fuel creating heat to perform work is a heat engine. Heat engines can be classified as external combustion, such as steam boiler, or internal combustion (IC). They can further be divided into the spark ignited (SI) engine or the compression ignited engine (CI). The particular heat engine we are interested about with this webpage, is the Compression Ignited heat engine. This style of engine has been most famously referred to as the Diesel engine, named after it’s inventor, Rudolph Diesel.

The compression ignited internal combustion engine has two main designs: the four stroke cycle, and the two stroke cycle engine. They are both very common throughout the world because of their high efficiency - compare with other types of heat engines. They burn a wide variety of hydrocarbon fuel, which is still common. Heavy fuel oil, intermediate fuel oil and marine diesel oil are the most common hydrocarbon fuel. The two stroke and four stroke titles refers to the mechanical action of the machinery, in particular the piston within the cylinder, to achieve the theoretical Carnot heat cycle, as define by the laws of thermodynamics.

Diesel engines are furthermore sub-divided into three categories: slow, medium and high speed. Slow speed are considered to be up to 300 rpm such as most big two stroke engines commonly found on ships. Medium speed engines dwell in the 300 - 900 rpm range. They are most common on smaller ships and power plants driving electrical generators and / or the propeller. High speed engines are the most common. Their high revolutions are ideal for driving vehicles such as busses and yachts. All Diesel trucks and vehicles on our roads use this class of Diesels, having an rpm over 900.

The two stroke engine...

It takes two strokes to complete a power cycle: one stroke down, one stroke up, resulting in one turn of the crankshaft. As oppose to the four stroke which must see the piston perform four strokes and two crank rotations, in order to achieve one cycle. The power stroke occurs at every down stroke which would lead us to believe the two stroke to be twice as efficient. On large plants, the two stroke is about 1.8:1 more powerful than a four stroke engine of similar weight. On smaller application, i.e. a bus, the power/pound gap closes, and the effective horsepower of similar sized engines will be about the same for the two and four stroke.

The two stroke is most commonly seen in application demanding a large power output, such as ships and electric generation plants. In the world of ships, these engines are large, especially tall; this is to accommodate the long stroke engine designers have come up with to increase the efficiency of the power stroke. Additionally, the large physical size of these power plants tends to slow them down.

This slow speed, around 100 rpm, has two benefits to an owner of a ships. The first, probably the most important, allows the engine to burn heavy fuel oil (HFO) efficiently. Although, the black, tar like substance can be used as fuel in both the four stroke and two stroke engine. This fuel is much cheaper, currently around $150USD/ton, compared to the "ordinary" refined diesel fuel oil (MDO), with the cost being around $300USD/ton.