Piston-Engine Based Power Plants

by Muhammad Hanif Sarwar

NFC(IET) Multan

The Bulk of material in these slide slides from Paul Breeze, Power Generation Technologies , Elsevier, 2005.

EE-415: Power Generation, Transmission and Distribution, Spring 2013

History

Same technology used today in oil

wells

The Lift Pump

Source: GEOS24705

History

First true steam engine by Thomas Newcomen (blacksmith) developed in 1712

Efficiency: 0.5%

Source: GEOS24705

History

First modern steam engine: James Watt, 1769 (patent), 1774 (prod.)

Source: GEOS24705

History

Source: GEOS24705

History

Double-action steam engine:

water-intensive, fuel-intensive – requires many stops to take on water and fuel.

Source: GEOS24705

Piston Engines

Piston engines or reciprocating engines are used throughout the world in applications ranging from lawn mowers to cars, locomotives, ships trucks, and for power and combined heat and power generation.

Engines vary in size from less than 1 kW to 65,000kW. They can burn a wide range of fuels including natural gas, biogas,

LPG, gasoline, diesel, biodiesel, heavy fuel oil and even coal. Small units:

Used for standby power or for combined heat, and power in homes and offices.

Usually cheap because they are mass produced Relatively low efficiencies Short lives.

Piston Engines

Larger standby units Used where supply of power is critical; in hospitals or to support highly

sensitive computer installations such as air traffic control

Medium-sized units Many commercial and industrial facilities use medium-sized piston engine-

based combined heat and power units for base-load power generation.

Large-sized units Can be used for base-load, grid-connected power generation Smaller units provide electricity to isolated communities Larger engines tend to be more expensive Large, megawatt scale engines are the most efficient prime movers

available, with simple cycle efficiencies approaching 50%.

Working Principal of a Piston Engine

In its most basic form, the piston engine comprises a cylinder sealed at one end and open at the other end.

A disc or piston which fits closely within the cylinder is used to seal the open end and this piston can move backwards and forwards within the cylinder.

This it does in response to the expansion and contraction of the gas contained within the cylinder.

The outside of the piston is attached via a hinged lever to a crankshaft. Movement of the piston in and out of the cylinder causes the crankshaft to rotate and this rotation is used to derive motive energy from the piston engine.

Working Principal of a Piston Engines

Reciprocating or Internal Combustion Engine

Stirling Engine

Types of Piston Engines Reciprocating or internal combustion engines

Spark Ignition type Compression or diesel type: High efficiency for power generation

applications but produces high level of atmospheric pollution particularly nitrogen oxide

Valves are employed to admit a mixture of fuel and air into the sealed piston chamber where it is burnt to generate energy.

External Combustion Engines or Stirling Engine Developed for specialized power generation Heat energy used to drive it is applied outside the sealed piston

chamber.

Piston Engine Types

Internal Combustion(IC) Engines

Spark Ignition Type The Otto cycle engine employs a spark to ignite a mixture of air and

fuel compressed by the piston within the cylinder. Explosive release of heat energy increases the gas pressure in the

cylinder, forcing the piston outwards as the gas expands. This explosion force on the piston turns the crankshaft

Compression Type Engines

The Otto cycle was modified by Rudolph Diesel in the 1890s.

Air is compressed by a piston to such a high pressure that its

temperature rises above the ignition point of the fuel which is then

introduced to the chamber and ignites spontaneously without spark.

A further subdivision depends on whether the engine utilises a

two- or a four-stroke cycle.

Two-stroke

The former is attractive in very small engines as it can provide relatively

high power for low weight.

For power generation, some very large engines also use a two-stroke

cycle.

Four-stroke

Most small- and medium-sized engines for power generation employ the

four-stroke cycle.

IC Engine Types

Four-Stroke Cycle

Intake Stroke Either air (diesel cycle) or a fuel and air mixture (Otto cycle) is drawn

into the piston chamber

Compression Stroke The gases in the cylinder are compressed

Power Stroke In the case of the Otto cycle, a spark ignites the fuel–air mixture at the

top of the piston movement creating an explosive expansion of the compressed mixture which forces the piston down again.

In the diesel cycle, fuel is introduced close to the top of the compression stroke, igniting spontaneously with the same effect.

Four-Stroke Cycle

Exhaust Stroke Exhaust gases are forced out of the piston chamber. In either case a

large flywheel attached to the crankshaft stores angular momentum generated by the power stroke and this provides sufficient momentum to carry the crankshaft and piston through the three other strokes required for each cycle.

The introduction of fuel and air, and the removal of exhaust is controlled by valves which are mechanically timed to coincide with the various stages of the cycle.

Four-Stroke Cycle

Four-Sroke Engines

Gasoline engines for automobiles typically have 4-8 cylinders Out-of-phase cylinders provide force to drive pistons through

compression phase and yield balanced power

Four-Stroke Engines

Two-Stroke Cycle

Intake and exhaust strokes are not separate. Instead fuel is forced

into the piston chamber (intake) towards the end of the power

stroke, pushing out exhaust gases through a valve at the top of the

chamber. A compression stroke is then followed by ignition of fuel

and a repeat of the cycle.

Two-stroke Cycle

Advantages: Simpler and cheaper Higher power-to-mass Each stroke is power stroke Smoother power in one-

cylinder engine.

Disadvantages: Some unburned fuel escapes

Three categories of engines : high-, medium-, and low-speed engines.

High-speed engines are the smallest and operate up to 3600 rpm. The largest slow-speed engines may run as slow as 58 rpm. Engine performance varies with speed. High-speed engines provide

the greatest power output as a function of cylinder size.

Engine Size and Speed

However the larger, slower engines are more efficient and last

longer. Thus the choice of engine will depend very much on

the application for which it is intended.

Large, slow- or medium-speed engines are generally more

suited to base-load generation but it may be more cost effective

to employ high-speed engines for back-up service where the

engines will not be required to operate for many hours each

year.

In addition to standby service or continuous output base-load

operation, piston engine power plants are good at load

following.

Engine Size and Speed

Spark Ignition Engines

They can burn a variety of fuels including gasoline, propane and landfill gas but the most common fuel for power generation applications is natural gas.

Most are four-stroke engines available in sizes up to around 6.5MW. With natural gas, the engine efficiency varies between 28% for smaller

engines and 42% for larger engines. An engine tuned for maximum efficiency will produce roughly twice as

much nitrogen oxides as an engine tuned for low emissions. Many natural gas engines are derived from diesel engines with 60–80%

of the output of the original diesel. More expensive than diesel engines but longer life and lower-

maintenance costs. Higher power can be achieved with a dual-fuel engine .

Dual Fuel Engines

Natural gas–air mixture is admitted to the cylinder during the intake

stroke, then compressed during the compression stroke.

At the top of the compression stroke the pilot diesel fuel is admitted

and ignites spontaneously, igniting the gas–air mixture to create the

power expansion.

Engine operates at close to the conditions of a diesel engine, with a

high-power output and high efficiency, yet with the emissions close to

those of a gas-fired spark-ignition engine.

Dual fuel engines operate with between 1% and 15% diesel fuel.

A dual fuel engine can also burn 100% diesel if necessary, with much

higher emissions.

Stirling Engine

Heat energy used to drive a Stirling engine is applied outside the

cylinders which are completely sealed.

Stirling engines often use helium or Hydrogen within cylinders.

A normal Stirling engine has two cylinders, an expansion cylinder

and a compression cylinder. The two are linked and heat is applied

to the expansion cylinder while the compression cylinder is cooled.

Careful balancing of the system allows the heat energy to be

converted into rotational motion.

Advantages: As heat energy is applied externally, hence in theory,

be derived from any heat source.

Stirling Engine

Stirling engines have been used to exploit solar energy and for biomass applications. However their use is not widespread.

Typical engines sizes in use and development range from 1 to 150kW.

Co-generation

The efficiency of piston-engine-based power generation varies from 25% for small engines to close to 50% for the very largest engines.

Between 50% and 75% of energy can be converted into combined heat and power (CHP) systems.

Between 30% and 50% of energy in exhaust gases can be used to generate medium-pressure steam or can be used to generate hot water.

The main engine case cooling system can capture up to 30% of the total energy input which could be passed through a heat exchanger to provide a source of hot water, although in some cases it can be used to produce low-pressure steam as well.

In the USA in 2000, there were 1055 engine-based CHP systems in operation with an aggregate generating capacity of 800MW

Diesel Engine-Steam Turbine Cogeneration Power Plant

Combined Cycle: Diesel Engine, HRSG (Heat Recovery Stream Generator) and Steam Turbine.

Macau China: Overall Efficiency: 50%

Cooling of a Diesel Engine

Air cooling used in small and

portable engines through fins.

In natural circulation systems,

water circulates due to

difference in density at

different temperatures.

Standby diesel power plants

up to 200 kVA use forced

circulation cooling system

In bigger plants, hot water is

cooled in a cooling tower and

re-circulated again. Make-up

water is therefore needed. Forced Circulation Cooling System.

Diesel Power Stations

Advantages The design and layout of the plant are quite simple. It occupies less space as the number and size of the auxiliaries is small. It can be located at any place. It can be started quickly and can pick up load in a short time. There are no standby losses. It requires less quantity of water for cooling. The overall cost is much less than that of steam power station of the same

capacity. The thermal efficiency of the plant is higher than that of a steam power

station. It requires less operating staff.

Diesel Power Stations

Disadvantages The plant has high running charges as the fuel (i.e., diesel) used is costly. The plant does not work satisfactorily under overload conditions for a

longer period. The plant can only generate small power. The cost of lubrication is generally high. The maintenance charges are generally high.

Diesel Power Plant Overview