four stroke engine

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 four stroke ...

four stroke engine

The four stroke is most commonly seen in application demanding a compact power plant. Ships need a large amount of power to propel it through the water, this is why the bigger ship, as previously mentioned, generally have a two stroke power plant. As illustrated by the pictures on this website, a person can begin to appreciate the physical size of these plants, especially the engine’s height. This height becomes a problem on many coastal ships, such ferries.

Take the British Columbia Ferry Corporation (BC Ferries) MV Queen of Cowichan, a double ender with a GT of 6,551 tons. This ferry, as do most ferry rely, on their ability to load and discharge a large amount of cars, transported on continuous decks. That creates a problem for the size engine the ferry needs. The power plant needs to fit below the main deck and still be able to go in shallow waters. The engines installed in the Queen of Cowichan are MAK 551 V12. They are a four stroke, turbo charged engine producing around 4,500 kW each. These engine stand less than twenty feet high and serve to illustrate the ideal application of a four stroke engine. A similar engine is pictured to the right.
Four stroke engines are more complicated than the two stroke and "pack" less horsepower per pound. The two stroke actually produces more power, around 1.8:1. But the four stroke consumes less oil and can operate safely at high speeds. The four stroke Diesel engine is probably the most prevalent engine in all the world. They can be found producing power for oil rigs in the North Atlantic or driving Volkswagen's New Beetle DTI.
The Diesel engine uses a light distillate fuel oil, commonly known as Diesel. This of course, is more expensive than the Heavy Fuel Oil (HFO) which is burned more efficiently by the two stroke engines. Recent engine design development has provided bigger four stroke power plants with the ability to burn HFO. This development now makes the four stroke engine an attractive option to ship owners. The newest shipping market to employ four stroke engines are the cruise ships. They use four or five engines driving generators to produce electricity which is used to power the main driving (electric) motors and the "hotel load" (lights, heat etc.). With the cruise ship, we can observe the point made earlier, the need to keep the power plant "small", this is to make room for paying passengers.

Many companies build four stroke Diesel engines. Caterpillar, Cummins, Detroit Diesel engines are the most popular engine manufacturer for highway trucks and smaller applications. Others like MAN (the company that first developed Rudolph Diesel’s engine), MAK, MTU, Wartsila, Deutz - just to name a few, offer a wide range of power plants often found in marine application. Four stroke engines have a power range of 2kW to 25,000kW. These can be found in many V style and inline configurations: V8, V12, V16, V20; the inline 6 being the more popular one.

The mechanics of it...

The four stroke is outwardly similar to it’s brother, the two stroke. But it’s inner working are quite different. One primary difference is the cylinder liner does not have the inlet ports machined into it. Another is the combustion air being drawn into the cylinder by the piston. This action is controlled by an inlet valve in the cylinder head, like the one to the right. With no need for a blower, this becomes the most radical difference between the four and two strokes engines and as such, the four strokes are called naturally aspirated engines.
The cycle comprises of: two crankshaft rotation and four change of direction by the piston. We will start with the inlet stroke at top dead center (TDC). See the diagram below for a visual representation.
At TDC the inlet valve is open allowing air to flow inside the cylinder. The downward action of the piston creates a vacuum which draws air into the cylinder. Shortly before the piston reaches bottom dead center (BDC), the inlet valve is closed. The piston at BDC begins it’s upward travel, beginning the compression stroke. The piston compresses the original air volume by about 17:1 to 23:1, to about 42 bars (~600 psi). The air’s volume reduction crushes the air molecule together and as result they build up heat.

1-2 suction, 2-3 compression, 3-4 injection (fuel)
4-5 expansion (power), 5-6 exhaust 

At 10 degrees before TDC injection of atomized fuel begins, it burns, ignited by the high temperature from to the air being compressed. Injection stops around about 35 degrees after TDC, depending on the engine’s load. The fuel continues to burn and expands which increase the cylinder temperature and pressure. Around 35 degrees after TDC injection stops, and shortly after maximum pressure is reached within the cylinder, this stage is commonly called Pmax. Pmax is anywhere between 180 bars to 195 bars (~2500 - 2800 psi); this is why Diesel engine construction is so robust, it is to withstand this pressure.

The pressure force acts upon the area of the piston crown, pushing it down. This downward force is transmitted to the crankshaft by the connecting rod, transforming it into a rotational force. At 35 degrees before BDC, the exhaust valve opens allowing the hot exhaust to escape, it's temperature is about 600 degrees Celsius. After BDC, the upward motion of the piston pushes the exhaust out of the cylinder through the exhaust valve. The air intake valve, 15 degrees before TDC, opens and overlaps the exhaust valve, this is to scavenge the cylinder of combustion products. Slightly after TDC, 10 degrees, the exhaust valve closes while the intake valve remains open to admit a fresh charge of air for compression. This completes one cycle of a four stroke engine.

two stroke

two stroke

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...

two stroke

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.
Slow speed, also allows a direct coupling of the propeller shaft to the crankshaft, eliminating the need for gearing and such. The reduction of associated gear serves to increase their power to weight ratio which usually equates to lower cost. Additionally, the two stroke engines have a less complicated design, namely no intake valves, this reduce the possibility of things failing. These attributes give the two stroke engine advantages over the four strokes, making it engine of choice for almost all large deep sea ships.
The most prominent builders of large, slow speed, two stroke engine are Sulzer of Switzerland, producer of the first big two stroke design, and Burmeister & Wain (B&W) of Denmark, the pioneers of the engine’s application to ship design. Other manufacturer also produce these types of engine, such as Kawasaki (Japan), Hyundai (Korea) and Fincantieri Naval (Italy) to name a few. Most shipbuilder build the Sulzer or B&W engines under license. To the land-lubber, the biggest builder of two stroke engine is the Detroit Diesel Corporation, now owned by Daimler Chrysler. "Gee Mees", as they are commonly known, can be found extensively in transit busses, fire trucks and many military applications. EMD, another division of General Motors, also builds two stroke engine which are very common on the North American coast and in locomotives. Two stroke engine have a wide variety of power output from the 353 Detroit Diesel with an output of less than 75 kW to a Sulzer RT-flex96C producing a 80,080 kW or about 109,000 bhp from fourteen cylinders turning at 102 rpm, not to be outdone MAN B&W has come out with the K108ME-C. This things is un-precedent big ! Like the Sulzer, its a 14 cylinder, but the bore, the area of the top of the piston, is 108cm, that's over a meter wide. It can produce a mind numbing 97,300 kW, over 130,000 hp, turning a propeller of almost 30 feet wide at 94 rpm. Both these engines are develop to power the latest and largest container ships, expected to carry 10,000 to 12,000 teu, or about 6000 tractor trailer trucks.

The components...

The heart of the two stroke engine comprises of a cylinder liner with inlet ports about 2/3 of the distance from the top of the liner. The cylinder head, also know as cylinder cover, contain the exhaust valve's) (pictured above). The piston within the cylinder is connected to the crankshaft by the connecting rod. On the larger engines, the power is transmitted to a cross head assembly first. The cross head slide up and down with the piston and transmit power from it to the connecting rod and crankshaft.
The two stroke engine is not a naturally aspirated engine. It depends a positive charge of air to fill and scavenge the cylinder. This is provided by a blower which charges the air box. The blower can be mechanically driven, such as on the Gee Mees or electric motor driven on the bigger engines. Most engines nowadays are supercharged (turbocharger) as well.

The chain reaction...

Describing the action of the piston within the cylinder generally start with the piston in the bottom dead center position (BDC), at the end of its travel downward before it’s travel upwards. With the piston, having covered the intake ports, travels upwards, compressing the trapped air from its original volume to a fraction of it, generally anywhere from 17:1 to 23:1 (gas engines are about 9:1) to about 42 bars (~600psi). The air’s volume reduction crushes the air molecule together and as a result they build up heat.

Slightly before top dead center (TDC), around 10 degrees, injection of atomized fuel begins, it burns, ignited by the high temperature from to the air being compressed. Injection last about 30 degrees, this varies depending on load. The fuel continues to burn and expands which increases the pressure and temperature within the cylinder. Around 15 degrees after TDC, combustion is complete and maximum pressure is reached within the cylinder, this stage is commonly called Pmax. Pmax is anywhere between 180 bars to 195 bars (~2500 - 2800 psi); this is why Diesel engine construction is so robust, it is to withstand this pressure.

The above timing diagram is a general one, of a two stroke engine design having inlet and exhaust ports in the cylinder liner or inlet port in the liner and valve exhaust in a cylinder cover.
The above timing diagram, of unknown origins, is most likely for a two stroke engine which has inlet and exhaust ports milled into the cylinder liner, with cylinder cover (head) being just a cover. 1-2 injection (fuel), 2-3 expansion (power), 3-5 exhaust 4-5 scavenging, 4-6 intake, 6-1 compression	The above timing diagram is for an opposed piston engine, the Doxford 76J4C. From Pounder's

The pressure force acts upon the area of the piston crown, pushing it down. This downward force is transmitted to the crankshaft by the connecting rod, transforming it into a rotational force. About 45 degrees before BDC and just prior to uncovering the inlet ports near the bottom of the cylinder, the exhaust valve open, releasing the pressure in the cylinder. The exhaust gases at this stage are about 600 degrees.
The continued downward travel of the piston uncovers the inlet ports, where positively charged air, 30-70 kPa (3-4 psi), provides a scavenging effect to drive exhaust gases out through the exhaust valve. This action not only clears the cylinder of spent gases but also cools the cylinder. Around 35 degrees after BDC the exhaust valve closes, and at 50 degrees after BDC the inlet ports are covered (closed).
With the inlet ports being covered by the upward traveling piston, the compression stroke begins again with a fresh charge of air. Thus the two stroke earns it’s name, one downwards (power stroke) and the upward stroke (compression stroke) complete one cycle.
The above process is best illustrated by the following diagram, courtesy of Rick Bogg's Merchant Marine website.