Piston (Reciprocating) Engine Power Plants
The internal combustion engine (ICE) has been refined and developed over the last 100 years for a wide variety of applications from tiny 1 cc engines powering model aircraft to gigantic marine engines with power outputs of tens of MegaWatts. The reciprocating engine with its compact size and its wide range of power outputs and fuel options is an ideal prime mover for powering electricity generating sets (gensets) used to provide primary power in remote locations or more generally for providing mobile and emergency or stand-by electrical power.
Generating sets are designed to work at fixed speeds because of the requirement to provide a fixed frequency AC voltage output. A rotor speed monitor provides an indication of the generator output frequency and this is fed back to control the fuel supply valve to keep the frequency constant.
The voltage is also proportional to the speed until the magnetic circuit saturates when rate of voltage increase, as the speed increases, slows dramatically .
The output power can be controlled by a thyristor controlled regulator which varies the ignition angle of the thyristor which in turn varies the average current into the load.
- Primary Power
Large Diesel generators are used for primary power applications
- Emergency Power
Small portable generators often used for emergency power may be petrol (gasoline) or diesel powered. Remote, unmanned applications are usually provide with an auto start-stop capability.
- Electric Traction
The first Diesel electric hybrid vehicle was patented in 1914 by Hermann Lemp. It used electric traction for the transmission system to avoid the use of complex gearing mechanisms necessary to deliver the power of the Diesel ICE to the wheels over the full range of train speeds since electric motors can operate over a wider speed range and can be more easily controlled. It used DC motors for this purpose and the DC power was provided by a DC generator driven by the Diesel engine. Modern Diesel electrics use AC machines to avoid the use of unreliable commutators and brushes in the motors and generators. Using Diesel electric power permits flexible routing and avoids the cost of the expensive infrastructure of overhead wires needed by pure electric trains. Electrical output power may be as low as 200 kW for a small passenger vehicle and up to 2 MW for a large freight train.
(See diagram for hybrid marine applications.)
Internal Combustion Engine Operating Principles
Internal combustion engines are constructed from one or more cylinders, each sealed at one end and open at the other, in which close fitting pistons can move up and down. (See diagram below) The engine derives its power from the burning of a compressed air-fuel mixture in each of the cylinders in succession. The fuel is ignited when the piston is at the top of its stroke and the expansion of the burning gas drives the piston downwards. The reciprocating motion of the pistons is converted to rotary movement by a crankshaft which delivers motive power to the desired application, in this case a generator. Air or an air-fuel mixture is introduced into the cylinder when the piston is at its lowest point and a flywheel on the crankshaft provides the momentum to drive the piston upwards to compress it.
The piston and connecting rod in a reciprocating engine form a large mass which is accelerated from zero to a very high speed and decelerated back to zero again with every revolution of the engine. (100 times per second in an engine operating at 6000 rpm.) This places immense forces on the moving parts of the engine.
Many methods of introducing the air and fuel into the cylinders, of controlling the ignition and of removing the exhaust gases have been developed over the years. The two main engine classes are spark ignition or Otto cycle engines and compression ignition or Diesel engines. Both of these types may be designed for four stroke or two stroke operation.
Simplified equations representing engine performance make the assumptions that the working substances are ideal gases, all processes are reversible and there is no friction.
The following idealised equation applies to both Otto (spark ignition) and Diesel (compression ignition) engines described below.
P= ηf ma N QHV (F/A) / nR
P = Engine power output
ηf = Fuel conversion efficiency
ma = Mass of air introduced into the cylinder(s) per cycle
N = Crankshaft rotational speed
QHV = Heating value of the fuel
(F/A) = Fuel mass flow rate / Air mass flow rate
nR = Number of crank revolutions per power stroke (2 for 4 stroke engines, 1 for 2 stroke engines)
From the equation we can see that output power is proportional to the mass of air passing through the engine (the capacity or swept volume of the cylinders), the speed of rotation, the energy content of the fuel and the rate at which it is consumed which can all be directly measured.
The output torque T is also proportional to the engine's capacity and the rate of fuel consumption and is given by:
The fuel conversion efficiency which affects both the engine power and the torque is more complex and dependent on the thermal and mechanical efficiency of the engine.
Energy Conversion Efficiency
The fundamental task of the an internal combustion engine is to convert chemical energy into mechanical energy by burning the fuel in the cylinder and the thermodynamic efficiency is a measure of how well it performs this job under ideal conditions. Practical systems however are subject to a variety of losses which conspire to reduce the overall efficiency of the engine in delivering mechanical energy to the crankshaft to surprisingly low values. Efficiencies can be as high as 50% or more for large diesel engines which make use of waste heat recovery systems but as low as 20% or 30% for simpler designs such as automotive power plants and small domestic electricity generating sets.
- Compression Ratio and Thermal Efficiency
Combustion efficiency can be improved by compressing the available oxygen and fuel molecules into a very small space which, together with the heat of compression, causes better mixing and evaporation of the fuel.
The theoretical thermodynamic efficiency of an Otto engine thus depends on the compression ratio of the working fluid as well as the nature of the fuel and is given by the standard equation:
Thermal efficiency = 1 - (1/rv)^γ-1
where rv is the compression ratio of the engine which is defined as the ratio between the volume enclosed by the cylinder and the piston when the piston is at bottom-dead-center (BDC), the volume enclosed by the cylinder and the piston when the piston is at top-dead-center (TDC).
Gamma (γ)is the ratio of specific heats at constant pressure (Cp) and constant
volume (Cv) of the working fluid ( for most purposes air is the working fluid,
and is treated as an ideal gas ). The gamma ratio for air is 1.4. The more complex the gas molecules, the lower the gamma. For the fuel mixture used in an internal combustion engine gamma would typically be between 1.15
The specific heat C is the amount of heat per unit mass required to raise the temperature by one degree Celsius. Thus:
C = Q / M * delta T
Where Q is the applied heat, M is the mass of the specimen and Delta T is the change in temperature which results. This assumes no phase change takes place since heat added or removed during a phase change does not change the temperature.
The thermal efficiency equation for an ideal Otto cycle is shown graphically below. It shows that the thermal efficiency and hence engine power increase with compression ratio however there is little improvement for compression ratios in excess of 17
The compression of the gas by the piston in the cylinder causes the gas temperature to rise and this temperature rise increases with increasing compression ratio. Since the compressed gas is a mixture of air with a volatile fuel, it may ignite spontaneously without a spark when the volatile fuel reaches its flash point before the piston has reached top of the compression stroke. This effect is called pre-ignition and limits the maximum compression ratio of a spark ignition engine to about 12:1
Compression ratios of spark ignition engines are typically in the range from 8:1 to 12:1
Diesel engines however which depend on the temperature rise caused by the compression to ignite the fuel rather than a spark, can, and must, work at much higher compression ratios. They can do this because the compressed gas is purely air and the fuel is not introduced until the air is already compressed.
For a given compression ratio, the Diesel engine is actually slightly less efficient than an Otto cycle engine but the Diesel more than compensates for this since it operates at much higher compression ratios.
Compression ratios of Diesel engines are typically in the range from 14:1 to 25:1
One downside of high compression engines is that the higher the
peak gas temperatures in the cylinder cause higher
the amounts of nitrogen oxides to be produced.
- Air / Fuel Ratio
The combustion process is a chemical reaction in which the fuel is oxidised (burned) by the oxygen in the air. For complete combustion to take place, a specific weight of air is required to oxidise all of the fuel without leaving any excess oxygen. The ratio of the weights of the air and the fuel required for complete combustion is called the stoichiometry ratio.
For petrol (gasoline) the stoichiometry air / fuel ratio is 14.7:1 and in an Otto cycle engine it is the task of the carburettor or the fuel injection system to maintain this ratio. If the air / fuel ratio is much higher than the stoichiometry value, as with a lean mixture, it is difficult to ignite the mixture with the spark plug. If the ratio is lower, as with a rich fuel mixture, some of the fuel remains unburnt and the engine efficiency suffers.
By contrast Diesel engines run at variable air / fuel ratios. This is because the ignition of the fuel is caused by the high temperature caused by compression and not by a spark.
Only a small amount of fuel is needed when the engine is idling but the combustion chamber is always full of pure air before the fuel is injected so that the air / fuel ratio can be as high as 60 or 100:1. As the load on the engine is increased, more fuel must be burned to provide the power and so the amount of fuel injected per cycle must be increased accordingly but the amount of air inducted into the cylinder per cycle is constant so that the air / fuel ratio is reduced.
Because the inefficient fuel - air mixing associated with Diesel engines results in incomplete combustion and the consequent production of soot particles when supplied with a rich fuel mixture, most Diesel engines need to run lean of the stoichiometric value of the fuel used. For the same swept volume, naturally aspirated Diesel engines can thus not burn as much fuel as equivalent Otto cycle engines which reduces slightly the efficiency advantage gained by their higher compression ratios.
- Energy Losses
All of the heat that comes out as exhaust or goes into the radiator is wasted energy.
Typically 35% of the applied heat energy is lost to the cooling system and slightly more through the exhaust. Incomplete combustion of the fuel gives rise to further losses. Friction accounts for another 5% to 6% of wasted energy and yet more energy is used to turn the various ancillary pumps, fans and generators needed to keep it going.
See also Heat Engines
Practical Power Output
Practical power output is limited by air flow restrictions due to limitations on the size and shape of the intake and exhaust paths, the fuel mixing efficiency, the flame propagation rate, friction, the ability of the mechanical components to withstand the high compression pressures in the cylinders and the extremely high inertial forces on the reciprocating parts including the connecting rods and the valve mechanisms.
The performance characteristics for a typical small engine which result from of all of these constraints are shown below.
The power and torque increase with engine speed but reach a peak and begin to tail off as these limitations begin to take effect. This is a major disadvantage for an automotive application which needs power and torque over a broad range of engine speeds, but not necessarily for a generator application which usually runs at constant speed.
- Spark Ignition Engines
The spark ignition engine was patented in 1876 by Nicolaus August Otto.
Until the 1980s, spark ignition engines used a carburettor to vaporise the fuel and mix it with the air. The air-fuel mixture is sucked into the cylinder by the downward movement of the piston and then compressed as the piston moves upwards. At the top of the cycle the mixture is ignited by a spark and the expanding, burning gas drives the piston down again. The intake and exhaust of the gases into and out of the cylinder is controlled by valve mechanisms at the top of the cylinder (the cylinder head) or by the movement of the piston past ports on the side of the cylinder.
The engine speed is controlled by a throttle (butterfly valve) which restricts the flow of the air-fuel mixture into the engine. The increased drag on the air flow caused by this mechanism hampers the engine breathing and so reduces its overall efficiency, particularly at slow speeds.
After 100 years of using carburettors in Otto cycle engines, fuel injection systems were introduced in the 1980s. With their much greater control over the combustion process they rapidly replaced the crude but reliable carburettor. They use electronic sensors to measure engine conditions such as air temperature and pressure and engine rpm. as well as the demand on the engine derived from the throttle position and use this information to deliver a precisely calculated charge of fuel into the engine through an injector. The fuel is injected at high pressure directly into the induction manifold or the cylinder or into a cavity in the cylinder head when the piston is near the top of its stroke and the air compression is almost complete. The fuel is atomised and mixes with the air prior to ignition by the spark. This system allows more precise timing and metering of the fuel flow, improving the combustion process, giving better efficiency and at the same time reducing harmful exhaust emissions.
- Diesel Engines
The compression ignition engine was patented in 1894 by Rudolf Diesel
Diesel engines are similar to Otto cycle engines but are designed to work at much higher compression ratios in order to achieve higher thermal efficiencies. To do this they aspirate only air during the compression cycle and the fuel is only introduced at the end of the compression cycle. In this way pre-ignition of the fuel is avoided since there is no fuel present during compression.
The high compression of the air causes its temperature to rise to over 700 to 900 degrees Celsius. Fuel oil is injected at high pressure into this hot air when the piston is at the top of its cycle causing the fuel charge to be atomised and ignition to occur immediately.
Engine speed is controlled by varying the fuel flow and the Diesel engine has no throttle valve to restrict the air flow. This makes it more efficient at low speeds than the Otto cycle engine.
Because of the high ignition temperatures achievable with high compression engines, Diesel engines can use much less volatile or less combustible fuels which in turn allows the engine to use a much wider range of fuels. The forced vaporisation of the fuel by the injector also assists in enabling the use of less volatile fuels.
Rudolf Diesel's original engine was designed to run on coal dust and later the French government, who were at the time exploring the possibility of using peanut oil as a locally produced fuel in their African colonies, pioneered biofuels by specifying peanut oil as fuel for the engine he demonstrated at the 1900 "Exposition Universelle" in Paris.
Large marine Diesels run on heavy fuel oil which is a waste product from the petroleum refining industry (sometimes called "bunker oil"). It is thick and viscous and difficult to ignite but safe to store and cheap to buy. Before use, the fuel must be heated to thin it out to aid vaporisation.
- Real and Illusory Efficiency Benefits
Because of their high compression ratios, Diesel engine deliver real efficiency improvements over the lower compression spark ignition engines, however this improvement amounts to only about 20% and does not account for the efficiency improvements of up to 40% claimed for the engine.
The other 20% improvement is due to the nature of the fuels. Both fuels have a similar energy density with petrol (gasoline) about 1 % better at 45.8 MegaJoules/kilogram (MJ/Kg) compared with Diesel oil with a density of 45.3 MJ/Kg. But Diesel oil is much denser than petrol with a density of 850 grams/litre which is about 18% denser than the more volatile petrol which has a density of only 720 grams/litre. Thus one litre or gallon of Diesel contains 17% more energy than than the equivalent volume of petrol.
When comparing the fuel consumption of automobile engines it is important to remember this.
If we ignore for the moment the 20% efficiency improvement due to the higher compression ratio of the Diesel engine, it would appear that Diesel engined cars are even more efficient, achieving an additional 17 % better mpg. However, this is only because fuel oil is sold by volume not by weight. When measured in miles per kilogram, the fuel consumption would be almost exactly the same.
- Otto / Diesel Comparison
The conversion efficiency for petrol engines (chemical energy to mechanical energy delivered to the crankshaft) is about 24% and about 32% for diesel engines
- Otto Cycle
- The relatively long period of one complete inlet stroke, available for fuel-air mixing means better mixing is possible in Otto cycle engines. This translates into better combustion control and fewer harmful emissions.
- Superior fuel mixing coupled with a relatively low compression ratio allows the spark ignition engine to run at high speeds.
- Higher speeds allow smaller engine size for the same power output.
(Power = Torque X rpm)
- Because of the lower compression ratio of the Otto cycle engine, it is subject to lower mechanical forces and so can be constructed from smaller and lighter components.
- Because the Otto cycle engine uses a volatile fuel mixed with air, the air-fuel mixture has a relatively low flash point. This limits the possible compression ratio which can be used. High compression ratios would raise the temperature of the air-fuel mixture to above its flash point causing the fuel to ignite prematurely before the piston reached the top of its compression stroke. This would tend to drive the piston in the reverse direction and is known as pre-ignition. Pre-ignition can however be minimised or avoided in fuel injection engines.
- The Otto cycle engine is less efficient than the Diesel engine because of its lower compression ratio.
- Usable fuels are limited to the more volatile hydrocarbons.
- Diesel Cycle
- Diesel engines are more efficient than the Otto cycle engines due to their higher compression ratios and are thus more economical to run.
- The combustion does not depend on natural vaporisation of the fuel so that a wide variety of less volatile and less combustible fuels can be used.
- Diesel engines tend to run cooler than spark ignition engines. Because of their higher fuel efficiency, they turn more of the fuel's heat energy into mechanical energy and reject less waste heat than spark ignition engines. For this reason Diesel engines have a lower risk of overheating if left idling for long periods of time. This makes them particularly suitable for marine and remote power generating applications where they may be required to run unattended for days at a time.
- High compression ratios are needed for compression ignition to take place. The detonation of the air-fuel mixture results in higher forces and shock loadings on the mechanical parts of the engine which must be bigger and heavier to accommodate these forces.
- The short duration available for fuel-air mixing at the top of the inlet stroke can result in poor fuel mixing and poor combustion characteristics. This in turn limits the possible engine speed and hence the possible power output.
- Because they run at a slower speed, Diesel engines must have a larger displacement (capacity) to produce the same power as a gasoline engine. This also means they must be bigger and heavier.
- Running at a slower speed also means that the Diesel engine must provide more torque to produce the same output power as Otto cycle engines.
- Because of the detonation of the fuel-air mixture, Diesel engines tend to be noisier than their Otto counterparts.
- Diesel engines are often supercharged to get more power out of the available capacity. This may reduce the overall weight of the engine but it adds to the cost and complexity.
- Four Stroke Engines
The four-stroke engine uses two engine revolutions for each power stroke, one to burn the fuel-air mixture and to clear out the exhaust gases and the other to reload the cylinder with the working fluid and compress it ready for ignition. Air flow through the engine is controlled by valve mechanisms in the cylinder head.
Lubricating oil is held in the crank case, isolated from the combustion chamber, and is pumped to the bearing surfaces via a separate pump.
Source: Derived from SIU Carbondale
- The Intake/Induction Stroke
The four-stroke cycle starts with the intake stroke when the piston is a the top of its travel. The inlet valve opens and as the piston moves downwards it sucks the working fluid (air or air-fuel mixture) into the cylinder under atmospheric pressure. The exhaust valve remains closed.
- The Compression Stroke
When the piston reaches the bottom of its travel the inlet valve is closed and the working fluid is compressed as the piston moves upwards.
- The Power Stroke
When the piston reaches the top of its travel, in the case of an Otto cycle engine, a spark ignites the air-fuel mixture initiating the power stroke in which the burning gas expands and forces the piston downwards. In Diesel engines, the fuel is injected into the compressed air which spontaneously ignites initiating the power stroke as in the Otto engine. Both inlet and exhaust valves remain closed.
- The Exhaust Stroke
As the piston passes through the end of its downwards travel, the exhaust valve is opened and the upwards movement of the piston expels the exhaust gases.
After the exhaust stroke the cycle starts again.
The precise timing of the opening and closing of the valves as well as the timing of the fuel ignition may be varied to improve the gas flow and combustion processes.
The engine only develops power during the power stroke. During the other three strokes the movement of the pistons is powered by the inertia of a flywheel on the crankshaft.
- Two Stroke Engines
The two-stroke engine uses only one engine revolution for each power stroke, the fuel air mixture is burned and the exhaust gases cleared out on the down stroke and the cylinder is recharged and the working fluid compressed during the upstroke. In its simplest form, as used in the spark ignition version, the two-stroke engine does not normally have separate valve mechanisms as in the four stroke engine. Instead the air and fuel flow in and out of the cylinder through ports (apertures) in the side of the cylinder wall which are opened or blocked by the passage of the piston which acts as a valve as it moves up and down past the ports in the cylinder wall. The inlet port is situated near the bottom of the cylinder and is connected to the crank case which is sealed and forms an essential part of the air-fuel management in this engine. Both sides of the piston are used in the two-stroke engine, the top side in the cylinder to provide the motive power and the underside in conjunction with the crank case to pump the air-fuel charge into the cylinder.
The exhaust port it situated further up the cylinder on the opposite side from the inlet port and is open to the atmosphere.
Source: Derived from PilotFriend Flight Training
Diesel versions, described below, are slightly more complex and usually have external valve mechanisms to control the air flow, rather than relying on the simple system of ports.
Using the crank case as a pressurisation chamber to pump the air-fuel mixture into the cylinder has consequences for the engine lubrication. The crank case can not hold both the volatile fuel mixture and heavy lubricating oil at the same time. Instead, oil must be mixed with the fuel to lubricate the crankshaft, connecting rods and cylinder walls.
- The Compression Stroke
Starting when the piston is at the bottom of its travel, both exhaust and inlet ports are uncovered. At this time a pressurised air-fuel mixture from the crank case enters the cylinder through the inlet port. As the piston moves upwards it first covers the inlet port then it covers the exhaust port situated further up the cylinder and the compression of the air fuel mixture commences and continues until the piston reaches the top of its stroke.
During this upward movement of the piston, a partial vacuum is created in the crank case underneath the piston drawing the air fuel mixture through a carburettor into the case past a reed type, non return valve ready to provide the next fuel charge.
The Diesel version does not depend on the pressurised crank case for providing the air-fuel charge. Since the Diesel breathes only air, aspiration is provided by a mechanically or turbine driven supercharger (See below) which forces air into the cylinder at the appropriate point in the cycle. This provides better scavenging and better control over the combustion and because the fuel does not enter the crank case, it can be sealed allowing the Diesel two-stroke engine to use conventional lubrication from an oil reservoir in the crank case.
- The Power Stroke
At the top of the compression cycle the air fuel mixture is ignited by a spark and the expansion of the burning gases pushes the piston downwards, turning the crankshaft.
At the same time the downward movement of the piston compresses the gases in the crank case on the underside of the piston.
As the piston nears the bottom of its stroke it first uncovers the exhaust port allowing the high pressure exhaust gases to be expelled. Further downwards movement of the piston towards the bottom of its travel uncovers the inlet port allowing the pressurised air-fuel mixture charge from the crank case to rush into the cylinder helping to sweep out any remaining exhaust gases in a process known as scavenging. The top of the piston is usually shaped to prevent the incoming fuel mixture from escaping out of the exhaust port. When the piston reaches the bottom of its travel the cycle starts again.
Note that the exhaust and intake both occur during the power stroke.
A flywheel on the crankshaft provides the momentum to complete the compression stroke.
- Four-Stroke Two-Stroke Comparison
- Four Stroke Engines
- Better control over the combustion process is possible due to more control possibilities with valve and ignition timing. This allows better fuel efficiency for the same compression ratio and better control of exhaust emissions.
- Better mixing of the fuel with the air due to the separate intake and compression cycle.
- Less power density than the two-stroke engine since there is only one power stroke every two engine revolutions.
- More complex and expensive to manufacture.
- Two-stroke Engines
- Because two-stroke engines have one power stroke every engine revolution they have a much lower weight and a significantly better power density than the four-stroke engine for the same power output.
- Two-stroke engines do not normally use complex external valve mechanisms, thus they have fewer moving parts and a much simpler, less expensive construction. This in turn lowers their weight further and allows them to run at very high speeds.
- Overall the two-stroke machine is a powerful, low cost, very simple, very light weight machine which is able to run at high speeds.
- Lubrication by mixing the oil with the fuel avoids the use of an oil sump and allows the engine to work in any orientation making it suitable for portable power tools.
- Though the two-stroke engine may have a greater power output, its actual efficiency is less than the equivalent four stroke engine. Inefficient fuel air mixing and inefficient scavenging leading to incomplete combustion, inefficient use of the fuel and unwanted exhaust emissions.
- Crank case pumping requires engine lubrication via oil mixed with the fuel. Can result in less efficient lubrication as well as unwanted burning of the lubrication oil during the combustion process creating further pollution.
- (Note: The Diesel two-stroke engine which breathes air and uses conventional lubrication does not suffer from either of the above two disadvantages.)
- Two-stroke Diesel engines normally need superchargers to achieve reasonable efficiency levels which adds considerably to the cost and complexity and precludes them from low cost applications.
Naturally aspirated four-stroke engines draw air into the cylinders by the downward movement of the piston which creates a partial vacuum inside the cylinders. The rate of air flow into the cylinder is limited by the maximum pressure difference between the pressure inside the cylinder and the outside atmosphere, namely 1 bar or 14.5 psi. This in turn limits the maximum power which can be extracted from the engine. The power output can however be increased by pumping air under pressure into the cylinders using a supercharger.
Similarly limitations apply to two-stroke engines. In this case, air is pumped into the cylinder from the pressurised crank case, also at low pressure but the power output can also be improved by supercharging.
The supercharger is essentially an air pump which may be gear driven from the engine crankshaft or turbine driven by the flow of the exhaust gases. In both cases the efficiency gain more than compensates for the energy used to drive the supercharger.
The DiesOtto Engine
Currently, automotive engineers are working on engines using a combination of Diesel and Otto engine technology.
The hybrid engine runs on petrol. On start-up, it runs in standard Otto mode, with spark plugs igniting petrol injected directly into the cylinder. Once the engine is warm and cruising, variable valve timing allows the compression ratio to be increased (See Miller cycle). The engine then switches to a more efficient Diesel mode and the spark plugs are deactivated. In this way it can achieve the advantages of both Otto and Diesel engines.
See also Hydrogen fuel
The problems of noxious exhaust emissions from fossil fuelled vehicles are well known. Fortunately electricity generation by piston power makes up a very small percent of electric power generation. See Fuel Sources
See also generators and external combustion engines
Electrical Energy Supply Overview