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Reciprocating engines

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The availability and economics of the prime mover fuel source and horsepower requirements frequently dictate that reciprocating internal combustion engines be selected to drive energy industry equipment.

Types of engine combustion

There are three main types of engine combustion.

All three types of engine combustion convert the chemical potential energy found in the fuel to mechanical kinetic energy. The combustion cycle and fuel type required to complete this task are what distinguish the three engine types from each other.

Two-stroke cycle engines or two-cycle engines complete their combustion cycle in two piston strokes that are accomplished with one revolution of the crankshaft. The two strokes are the power and compression strokes. The two-stroke engine is unique because it does not control the release of exhaust or the admission of an air/fuel mixture into the cylinder with a traditional valve arrangement, one intake and one exhaust.

Two stroke cycle

As shown in Fig. 1, the process of filling the cylinder with an air/fuel mixture and exhausting the burned gases occurs almost simultaneously near the end of the power stroke. As the piston moves downward during the power stroke, first the exhaust port is uncovered and then the intake port is uncovered. These ports allow the exhaust to escape under its own pressure and then be displaced by the incoming air/fuel mixture. Because some of the air/fuel mixture can be lost in this process, some two-stroke engines are equipped with fuel injection systems that raise their fuel efficiency close to that of a four-stroke engine. The compression stroke compresses the air/fuel mixture that is then ignited by a spark plug. The burning of the air/fuel mixture pushes the piston through the power stroke. As the power stroke reaches its end, the cycle begins again.

Lubricating a two-stroke engine is also unique compared with other engines because the engine does not have a traditional valve arrangement that separates the crankcase from the combustion chamber. Lubrication of the engine is accomplished by an auxiliary lubricator that injects a prescribed amount of oil into critical areas. Another method, commonly used on small two-stroke engines, is to combine a prescribed amount of oil with the fuel so that it lubricates the engine as the air/fuel mixture passes through the crankcase.

Most two-stroke engines are the slow to mid-speed variety (400 to 700 rpm), primarily because of lubricating concerns and because the crankshaft is turning only once per cycle. Some of the largest engines in the world are two-stroke engines. As engine size and power increase, the size and weight of the internal components also increase, which limits their ability to reciprocate quickly. Therefore, the two-stroke engine design works well for large high-power engines with its slower speed and fewer strokes. There are two-stroke engine models rated well in excess of 5,000 brake hp (BHP).

Four stroke cycle

A four-stroke cycle engine is commonly called a four-cycle engine. Four-cycle engines complete their combustion cycle in four strokes of the piston that are accomplished with two revolutions of the crankshaft, as shown in Fig. 2. The four strokes are:

  • Intake
  • Compression
  • Power
  • Exhaust

The four-cycle engine controls the release of exhaust and the admission of a fresh charge or air/fuel mixture using a traditional valve arrangement, one intake and one exhaust. These valves are located in the engine head or block and are typically actuated with camshaft(s) and push rods.

The intake stroke moves the piston to create an expansion of the combustion chamber volume. The intake valve is opened during the intake stroke, and an air/fuel mixture is drawn into the cylinder. The compression stroke compresses the air/fuel mixture, which increases the energy potential of the mixture. The compressed air/fuel mixture is then ignited by a spark plug, and the burning of the air/fuel mixture pushes the piston through the power stroke. The exhaust valve then opens, and the piston pushes the exhaust gases out of the cylinder through the exhaust stroke. One revolution of the crankshaft completes the intake and compression strokes. The second revolution of the crankshaft completes the power and exhaust strokes, ending the four-stroke cycle.

A four-cycle engine is lubricated through a pressurized oil system. Typically, oil is stored in an oil pan or sump. An oil pump draws oil from the sump and sends pressurized oil through a filter and then to all engine bearings and moving parts. The oil then returns to the oil sump. In a four-cycle engine, the fuel system and air/fuel mixture are always kept separate from the crankcase and oil.

Most four-cycle engines used in the energy industry are the mid- to high-speed variety (900 to 2000 rpm). Higher speeds are allowed with four-cycle engines because of the pressurized lubrication systems and smaller reciprocating components. The four-cycle design works well for most applications below 20,000 brake horsepower (BHP), where speed and fuel efficiency are concerns. Higher speeds are preferred in numerous energy industry applications. For compression applications, higher speeds mean more turns for the compressor and more gas transferred in a given time. For electrical power generation applications, higher speeds mean a more steady production of electricity at any given frequency. Typically, four-cycle engines used in the energy industry are rated up to about 20,000 BHP. The numbers in Fig. 2 indicate the beginning for each of the four main events of the four-stroke engine cycle. The intake stroke is designated 1, where volume is increasing and pressure remains low. The rest of the cycle is illustrated as a combination of pressure and volume changes.

Diesel cycle

The diesel cycle also completes its work in four strokes, making it a four-cycle engine. However, the difference between a diesel and a typical four-cycle engine is the cause of the air/fuel mixture ignition. Diesel engines do not have spark plugs like two and four-cycle engines. The diesel engine uses a high compression ratio and the resulting heat for the ignition of the diesel/air mixture. Natural gas cannot be used in a diesel cycle because ignition temperature is too high.

The intake stroke moves the piston to create an expansion of the combustion chamber volume. The intake valve is opened during the intake stroke, and fresh air is drawn into the cylinder. The compression stroke compresses the fresh air, which increases the kinetic energy of the air, resulting in a high temperature. A fuel injection valve then injects a prescribed amount of diesel fuel into the cylinder. The injected fuel mixes with the hot compressed air in the cylinder. The heat ignites the fuel without a spark, and the burning of the mixture pushes the piston through the power stroke. The exhaust valve then opens, and the piston pushes the exhaust gases out of the cylinder through the exhaust stroke. One revolution of the crankshaft completes the intake and compression strokes. The second revolution of the crankshaft completes the power and exhaust strokes, ending the four-stroke diesel cycle.

Four-stroke diesel engines can have compression ratios beyond 22:1, double that of a nondiesel engine. Air in the cylinder is subjected to this very high compression ratio. The result is a buildup of heat in the compressed volume of air. The heat generated by compressing air to such an extent results in enough heat to ignite the fuel. The temperatures in a cylinder can be in excess of 1,000°F.

A benefit to diesel technology is high mechanical efficiencies. Diesel engines can have an efficiency of > 40%. This level of efficiency is caused by the high temperatures and unique ignition of the diesel/air mixture. Diesel engines are commonly found in very large high-powered, slower-speed applications like seaworthy cargo ships. Diesel engines are also commonly found in all of the same applications that four-cycle non-diesel engines are found. The major differences are the fuel system and availability of that fuel.

Naturally aspirated vs. turbocharged

Regardless of the type of engine selected to be a prime mover, one issue that needs to be addressed is whether the engine will be naturally aspirated or turbocharged. Either option has advantages and disadvantages.

Naturally aspirated engines breathe directly from the environment, which means that air enters the cylinder under atmospheric pressure. During the intake stroke, the open area in the combustion chamber expands, resulting in reduced pressure. In a naturally aspirated engine, the atmospheric pressure causes the intake air to flow naturally from high to low pressure and into the combustion chamber. Because a naturally aspirated engine relies on atmospheric pressure, it is more prone to being affected by altitude changes. Power may be lost at higher altitudes because of the less dense air and lower atmospheric pressure.

In a turbocharged engine, the intake air is compressed with a turbine that is driven by exhaust gases. A turbocharged engine breathes the compressed air that is at a higher pressure than atmospheric pressure. Because the intake air is compressed, a more dense air enters the cylinder during the intake stroke. The effect is more power from the same cylinder size because more fuel is needed to match the extra air molecules of the dense compressed air. More fuel and air in the cylinder means more potential energy to compress and burn. Turbocharged engines have a better chance of maintaining power levels at higher altitudes. This stability results because the engine breathes compressed air, leaving the work of the altitude increase to the turbocharger and not the engine.

However, two advantages of the naturally aspirated engines are fewer parts and less complex applications. The naturally aspirated engine does not have a turbocharger and all of the related equipment, including:

  • An intercooler
  • Auxiliary water piping
  • Turbocharger lube oil plumbing
  • Waste-gate valves
  • Bypass valves

Another advantage is the ability to burn a wider range of fuels. Because the firing pressures of naturally aspirated engines are lower than turbocharged engines, naturally aspirated engines are able to burn richer fuels [i.e., those with higher calorific content (Btu/ft3)] and maintain power levels. The lower firing pressures also allow a longer engine life cycle. The longer life cycle and less complex fuel system sometimes make the naturally aspirated engine preferable for remote, minimal-maintenance applications in which ease of maintenance is required.

Exhaust emissions

Deterioration of the atmosphere from gaseous pollutants is an important environmental issue. All engine types produce emissions of some form and are possibly subject to emission regulation. Local, state, and national governments continue to enact stricter exhaust emission requirements to reduce atmospheric deterioration. Two ways to reduce atmospheric deterioration are:

  • To limit the power of engines
  • To require low emissions levels from an engine. Many applications are subject to emission control regulations.

The six main emission pollutants are classified into six different categories:

  • NOx (oxides of nitrogen)
  • CO (carbon monoxide)
  • HC (hydrocarbons)
  • SOx (oxides of sulfur)
  • CHO (aldehydes)
  • PM10 (particulate matter 10 μm and smaller)

NOx (oxides of nitrogen)

NOx consists of nitric oxide (NO) and nitrogen dioxide (NO2) molecules formed when nitrogen (N2) and oxygen (O2) react with each other. This reaction requires a high combustion temperature and the presence of N2 and O2 in the combustion chamber as a fuel is burned. NO2 is harmful to humans and animals because it reduces breathing capacity and the ability of the blood to carry O2. When NOx is exposed to sunlight, it acts as a precursor in the formation of harmful lower atmosphere ozone (O3).

CO (carbon monoxide)

Carbon monoxide (CO) is formed by incomplete combustion of a fuel. Incomplete combustion occurs when there is insufficient O2 to complete the combustion of the fuel molecule or when the combustion is quenched near a cold surface in the combustion chamber. Carbon monoxide is a poisonous gas that causes nausea, headache, and fatigue; in heavy enough concentrations, CO can even cause death. In the upper atmosphere, CO reacts with O3 to produce CO2. This reduces the O3 in the upper atmosphere. Ozone in the upper atmosphere screens harmful sunrays from reaching the Earth’s surface.

HC (hydrocarbons)

The burnable components of any fuel are the hydrocarbons (HC). A small fraction of HCs will pass through the combustion chamber and retain their original form in the exhaust. The nonmethane HCs (any HC other than methane) can react with the NOx in the lower atmosphere, acting as a precursor for the formation of photochemical smog.

SOx (oxides of sulfur)

Oxides of sulfur are formed when sulfur-containing compounds are oxidized in the combustion chamber. These compounds can be found in the lube oil or in the fuel of an engine. Oxides of sulfur enter the atmosphere and combine with water to form sulfuric acid. These acids return to Earth as acid rain.

CHO (aldehydes)

Aldehydes (CHO) are formed during the combustion of liquid fuels and lube oil in an engine. Therefore, CHO levels from gas-fueled engines are extremely low compared with liquid-fueled engines like diesel. Aldehydes contribute to photochemical smog and eye irritation.

Particulate matter is also formed during the combustion of liquid fuels and engine lubricants. Particulate matter is often seen as black smoke coming from diesel engines. Some regulatory agencies have labeled particulate matter from diesel engines as a possible carcinogen.

Emission control

A popular method for reducing and controlling emission levels is to use catalyst reduction and air/fuel ratio control. A catalyst is a substance that promotes a chemical reaction to convert emissions into harmless, naturally occurring compounds without chemically changing itself. A catalyst operates in one of two ways:

  1. It oxidizes (oxidation catalyst)
  2. It reduces (reduction catalyst) the emission component

Catalyst reduction can be applied to either a rich-burn or lean-burn engine. The discussion of rich-burn engines vs. lean-burn engines applies primarily to gas-fueled engines. Liquid-fueled engines are generally rich-burn engines.

Rich burn engines

Rich-burn engines operate at near-stoichiometric combustion at a point where the air/fuel ratio is nearly 16:1. Stoichiometric combustion occurs when there is a correct proportion of O2 and fuel so that they completely react in the combustion process, thus maximizing fuel efficiency and capturing the most power from the fuel. However, operating at near-stoichiometric combustion produces the highest levels of emissions. Today, most rich-burn engines entering the field use some form of catalyst aftertreatment for emissions. Catalyst performance in a rich-burn engine depends on the composition of the exhaust entering the catalytic converter.

Many rich-burn engines with a catalyst use an air/fuel ratio controller. An air/fuel ratio controller generally monitors the amount of O2 in the exhaust stream, compares it with a desired set point, and then changes the air/fuel ratio accordingly. When the air/fuel ratio of an engine is controlled and held at an ideal setting, the chemical reaction occurring inside the catalyst is maximized. Many catalysts can have a reduction efficiency of > 90%.

Lean burn engines

Lean-burn engines operate well beyond a 16:1 stoichiometric air/fuel ratio. They can have air/fuel ratios of 24:1 up to 32:1. A lean-burn engine with a 32:1 air/fuel ratio has twice as much O2 than is needed for the fuel in the combustion chamber. However, having the excess O2 ensures that fuel molecules will have a better opportunity to react with the needed O2 for complete combustion of the fuel. Lean-burn engines consistently have lower NOx and CO emissions primarily because of the excess O2 in the combustion chamber and lower exhaust temperatures, which inhibit the formation of NOx emissions. Generally, lean-burn engines do not use catalyst aftertreatment for emissions because the emission levels usually are low enough to meet regulations. An air/fuel ratio controller is typically not used with a lean-burn engine because the exhaust composition does not need to be maintained for a catalytic converter. However, if further emission reduction is needed, a selective catalyst can be used with a lean-burn engine. A common selective catalyst system injects ammonia urea (NH3) to react with NOx. The ammonia is consumed in the reaction to leave the emissions of N2 and H2O. These applications need to be considered with caution. A selective catalyst reduction system is expensive compared with a rich-burn oxidizing catalyst. A selective catalyst reduction system also requires on-site storage of hazardous ammonia.

Rich burn vs. lean burn engines

Fig. 3 illustrates NOx and CO emission output versus excess air/fuel ratio and excess air ratio for natural gas engines. Note the impact that air/fuel ratios have on rich-burn engine emissions compared with the impact on lean-burn engines. Air/fuel ratio controllers are very important in rich-burn engine applications in which emissions need to be controlled.

Engine families and interchangeability

Regardless of the engine type or engine manufacturer, families and parts interchangeability will most likely play a role in a particular product lineup. Engine manufacturers generally group their products into engine families that are designated by:

  • Engine
  • Bore
  • Block type

Sometimes, they are separated to service different markets or different horsepower ranges. Within each engine family are parts interchangeability and commonality from both a manufacturing and a marketing standpoint. Engine manufacturers make parts common within a family so that the same part can be used in multiple engine models. Thus, the manufacturer has to have tooling and engineering support for only one part rather than multiple parts within the same family.

Marketing and sales departments use parts commonality and interchangeability as selling points. For example, gas compression sites may have more than one engine on site, and a power generation distributor may have more than one engine in its lease fleet. If the engines are from the same family and have commonality between them, then many parts may be shared between the multiple engines, so a service truck or a parts warehouse needs to stock fewer parts. An on-site emergency parts supply will also be smaller because the few parts kept can support all the engines. Common parts between engines are usually:

  • Pistons
  • Cylinder liners
  • Connecting rods
  • Bearings
  • Valve drive components

Sometimes, within a given family, cylinder heads will be the same and interchangeable. Cylinder head interchangeability can be particularly useful if an engine fleet has many different engine sizes but is from the same family. Heads can be taken off one engine that is being upgraded and then, after servicing, put on a different engine.

Engine fuels

Engine fuel can be in a liquid state like diesel, gasoline, and jet fuel. Alternately, it can be in a gaseous state such as natural gas, propane vapor, and Biogas.

Liquid fuels allow on-site fuel storage for applications such as road, sea or air vehicles. Liquid fuels can also be used for applications in which sites are too remote for a gas utility to reach such as remote power generation. Liquid fuels provide the convenience of having fuel storage built into the equipment. A rental fleet may use base tanks under their units to allow easy relocation. Gaseous fuels, such as natural gas, make it convenient to connect to a utility gas supply to fuel large stationary engines. In these applications, fuel demand is too great for on-site fuel storage to be practical. Gaseous fuels such as biogas are a byproduct of trash decomposition or sewage treatment plant processes. Using this byproduct as a gaseous engine fuel captures energy that would normally be flared off and wasted.


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See also

Prime movers

Gas turbine engines