Spark-ignition engine

1.INTRODUCTION: The term spark-ignition engine refers to internal combustion engines, generally petrol engines, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug. This is in contrast to compression-ignition engines, typically diesel engines, where the heat generated from compression together with the injection of fuel is enough to initiate the combustion process, without needing any external spark.

Spark-ignition engines are commonly referred to as "gasoline engines" in America, and "petrol engines" in Britain and the rest of the world. However, these terms are not preferred, since spark-ignition engines can (and increasingly are) run on fuels other than petrol/gasoline, such as autogas (LPG), methanol, ethanol, bioethanol, compressed natural gas (CNG), hydrogen, and (in drag racing) nitromethanethe working cycle of both spark-ignition and compression-ignition engines may be either two-stroke or four-stroke.A four-stroke spark-ignition engine is an Otto cycle engine. It consists of following four strokes: suction or intake stroke, compression stroke, expansion or power stroke, exhaust stroke. Each stroke consists of 180 degree rotation of crankshaft rotation and hence a four-stroke cycle is completed through 720 degree of crank rotation. Thus for one complete cycle there is only one power stroke while the crankshaft turns by two revolutions See also Compression-ignition engine Multifuel engine.

CHAPTER -2ENGINE & WORKING PRINCIPLE

A heat engine is a machine, which converts heat energy into mechanical energy. Thecombustion of fuel such as coal, petrol, diesel generates heat. This heat is supplie toaworking substance at high temperature. By the expansion of this substance in suitabemachines, heat energy is converted into useful work. Heat engines can be further divided intotwo types:

(i) External combustion and(ii) Internal combustion.

Ina steam engine the combustion of fuel takes place outside the engine and the steamthus formed is used to run the engine. Thus, it is known as external combustion engine.In thecase of internalcombustion engine, the combustion of fuel takes place inside the enginecylinder itself.

The IC engine can be further classified as: (i) stationary or mobile, (ii) horizontal or verti-cal and (iii) low, medium or high speed. The two distinct types of IC engines used for eithermobile or stationary operations are: (i) diesel and (ii) carburettor.

1.1Spark Ignition (Carburettor Type) IC Engine

In this engine liquid fuel is atomised, vaporized and mixed with air in correct proportionbefore being taken to the engine cylinder through the intake manifolds. The ignition of themixture is caused by an electric spark and is known as spark ignition.

1.2Compression Ignition (Diesel Type) IC Engine In this only the liquid fuel is injected in the cylinder under high pressure.

1.3.CONSTRUCTIONAL FEATURES OF IC ENGINE:

1. Cylinder 2. Spark plug3. Cylinder head 4. inlet valve5. Piston

6. exhaust valve7. Connecting rod 8. cooling fins9. crank and crankshaft 10.carburretor11.Crank pin and bearings 12.flywheel13.Piston rings 14.main bearings15.Gudgeon pin 16.valve mechanism

1. CYLINDER:The cylinder of an IC engine constitutes the basic and supporting portion of the engine powerunit. Its major function is to provide space in which the piston can operate to draw in the fuelmixture or air (depending upon spark ignition or compression ignition), compress it, allow itto expand and thus generate power. The cylinder is usually made of high-grade cast iron. Insome cases, to give greater strength and wear resistance with less weight, chromium, nickel.

2. SPARK PLUG:A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto. The spark plug's metal shell is screwed into the engine's cylinder head and thus electrically grounded.

Spark plugs may also be used for other purposes; in Saab Direct Ignition when they are not firing, spark plugs are used to measure ionization in the cylinders – this ionic current measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire measurement function.Spark plugs may also be used in other applications such as furnaces wherein a combustible fuel/air mixture must be ignited. In this case, they are sometimes referred to as flame igniters.

2.1.SPARK PLUG CONSTRUCTION:

A spark plug is composed of a shell, insulator and the central conductor. It passes through the wall of the combustion chamber and therefore must also seal the combustion chamber against high pressures and temperatures without deteriorating over long periods of time and extended use.Spark plugs are specified by size, either thread or nut (often referred to asEuro), sealing type (taper or crush washer), and spark gap.

2.2.PARTS OF THE SPARK PLUG:

Terminal

The top of the spark plug contains a terminal to connect to the ignition system. The exact terminal construction varies depending on the use of the spark plug. Most passenger car spark plug wires snap onto the terminal of the plug, but some wires have eyelet connectors which are fastened onto the plug under a nut.

Insulator

The main part of te insulator is typically made from sintered alumina, a very hard ceramic material with high dielectric strength, printed with the manufacturer's name and identifying marks, then glazed to improve resistance to surface spark tracking.

Ribs

By lengthening the surface between the high voltage terminal and the grounded metal case of the spark plug, the physical shape of the ribs functions to improve the electrical insulation and prevent electrical energy from leaking along the insulator surface from the terminal to the metal case.

Insulator tip

On modern (post 1930s) spark plugs, the tip of the insulator protruding into the combustion chamber is the same sintered aluminium oxide (alumina) ceramic as the upper portion, merely unglazed. It is designed to withstand 650 °C (1,200 °F) and 60 kV.

The dimensions of the insulator and the metal conductor core determine the heat range of the plug. Short insulators are usually "cooler" plugs, while "hotter" plugs are made with a lengthened path to the metal body, though this also depends on the thermally conductive metal core.

Seals

Because the spark plug also seals the combustion chamber or the engine when installed, seals are required to ensure there is no leakage from the combustion chamber. The internal seals of modern plugs are made of compressed glass/metal powder, but old style seals were typically made by the use of a multi-layer braze.

Metal case/shell

The metal case/shell (or the jacket, as many people call it) of the spark plug withstands the torque of tightening the plug, serves to remove heat from the insulator and pass it on to the cylinder head, and acts as the ground for the sparks passing through the central electrode to the side electrode. Spark plug threads are cold rolled to prevent thermal cycle fatigue.

Central electrode

Central and lateral electrodes

The central electrode is connected to the terminal through an internal wire and commonly a ceramic series resistance to reduce emission of RF noise from the sparking. Non-resistor spark plugs, commonly sold without an "R" in the plug type part number, lack this element to reduce electro-magnetic interference with radios and other sensitive equipment. The tip can be made of a combination of copper, nickel-iron, chromium, or noble metals.

Side (ground, earth) electrode

The side electrode (also known as the "ground strap") is made from high nickel steel and is welded or hot forged to the side of the metal shell. The side electrode also runs very hot, especially on projected nose plugs. Some designs have provided a copper core to this electrode, so as to increase heat conduction. Multiple side electrodes may also be used, so that they don't overlap the central electrode.

Spark plug gap

Gap gauge: A disk with sloping edge; the edge is thicker going counter-clockwise, and a spark plug will be hooked along the edge to check the gap.

Spark plugs are typically designed to have a spark gap which can be adjusted by the technician installing the spark plug, by bending the ground electrode slightly. The same plug may be specified for several different engines, requiring a different gap for each. Spark plugs in automobiles generally have a gap between 0.6–1.8 mm (0.024"–0.070"). The gap may require adjustment from the out-of-the-box gap.

Variations on the basic design

Over the years variations on the basic spark plug design have attempted to provide either better ignition, longer life, or both. Such variations include the use of two, three, or four equally spaced ground electrodes surrounding the central electrode.

Surface-discharge spark plug

A piston engine has a part of the combustion chamber that is always out of reach of the piston; and this zone is where the conventional spark plug is located. A Wankel engine has a permanently varying combustion area; and the spark plug is inevitably swept by the tip seals. Clearly, if a spark plug were to protrude into the Wankel's combustion chamber it would foul the rotating tip; and if the plug were recessed to avoid this, the sunken spark might lead to poor combustion.

Tip protrusion Different spark plug sizes. The left and right plug are identical in threading, electrodes, tip protrusion, and heat range. The centre plug is a compact variant, with smaller hex and porcelain portions outside the head, to be used where space is limited. The rightmost plug has a longer threaded portion, to be used in a thicker cylinder head.

CHAPTER -2 FOUR STROKE SI ENGINE

this gasoline is mixed with air, broken up into a mist and partially vaporized in a carburettor (Fig. 5). The mixture is then sucked into the cylinder. There it is compressed by the upward movement of the piston and is ignited by an electric spark. When the mixture is burned, the resulting heat causes the gases to expand. The expanding gases exert a pressure on the piston (power stroke). The exhaust gases escape in the next upward movement of the piston. The strokes are similar to those discussed under four-stroke diesel engines. The various temperatures and pressures are shown in Fig. 6. The compression ratio varies from4:1 to 8:1 and the air-fuel mixture from 10:1 to 20:1. The first petrol combustion engine (one cylinder, 121.6 cm3 displacement) was prototyped in 1882 in Italy by Enrico Bernardi. In most petrol engines, the fuel and air are usually pre-mixed before compression (although some modern petrol engines now use cylinder-direct petrol injection). The pre-mixing was formerly done in a carburetor, but now it is done by electronically controlled fuel injection, except in small engines where the cost/complication of electronics does not justify the added engine efficiency. The process differs from a diesel engine in the method of mixing the fuel and air, and in using spark plugs to initiate the combustion process. In a diesel engine, only air is compressed (and therefore heated), and the fuel is injected into very hot air at the end of the compression stroke, and self-ignites.

Fig.1. Principle of operation of four-stroke petrol engine A four-stroke engine (also known as four cycle) is an internal combustion (IC) engine in which

the piston completes four separate strokes while turning a crankshaft. A stroke refers to the full

travel of the piston along the cylinder, in either direction. The four separate strokes are termed.

Each of its stroke consists of 180 degree rotation of crankshaft rotation and hence a four-

stroke cycle is completed through 720 degree of crank rotation. Thus for one complete cycle

there is only one power stroke while the crankshaft turns by two revolutions.

Suction stroke

Compression stroke

Expansion stroke

Exhaust stroke

1.Suction stroke: This stroke of the piston begins at top dead center (T.D.C.) and ends at

bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position

while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into

the cylinder through its downward motion.

2.Compression stroke: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.

3.expansion stroke(power): This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to the crankshaft.

4.Exhaust stroke: During the exhaust stroke, the piston once again returns from B.D.C. toT.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.

fig2.1 Four Stroke SI Engine

2.1. Otto cycle:

An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine.It is the thermodynamic cycle most commonly found in automobile engines.

Compression

Stroke

PowerStroke

Exhaust

Stroke

A I R

CombustionProducts

Ignition

IntakeStroke

FUEL

Fuel/AirMixture

Pressure-Volume diagram

Temperature-Entropy diagram

The idealized diagrams of a four-stroke Otto cycle Both diagrams:the  intake (A)  stroke is performed by an isobaric expansion, followed by an adiabatic  compression(B) stroke. Through the combustion of fuel, heat is added in an a constant volume (isochoric process) process, followed by an adiabatic expansion process power (C)  stroke. The cycle is closed by the  exhaust (D)  stroke, characterized by isochoric cooling and isentropic compression processes.

The Otto cycle is a description of what happens to a mass of gas as it is subjected to changes of pressure, temperature, volume, addition of heat, and removal of heat. The mass of gas that is subjected to those changes is called the system. The system, in this case, is defined to be the fluid (gas) within the cylinder. By describing the changes that take place within the system, it will also describe in inverse, the system's effect on the environment. In the case of

the Otto cycle, the effect will be to produce enough net work from the system so as to propel an automobile and its occupants in the environment.

The Otto cycle is constructed from:Top and bottom of the loop: a pair of quasi-parallel and isentropic processes (frictionless, adiabatic reversible).Left and right sides of the loop: a pair of parallel isochoric processes (constant volume).

The isentropic process of compression or expansion implies that there will be no inefficiency (loss of mechanical energy), and there be no transfer of heat into or out of the system during that process. Hence the cylinder, and piston are assumed impermeable to heat during that time. Work is performed on the system during the lower isentropic compression process. Heat flows into the Otto cycle through the left pressurizing process and some of it flows back out through the right depressurizing process. The summation of the work added to the system plus the heat added minus the heat removed yields the net mechanical work generated by the system.

The processes are described by:

Process 0–1: it is a mass of air is drawn into piston/cylinder arrangement at constant pressure.

Process 1–2: it is an adiabatic (isentropic) compression of the air as the piston moves from bottom dead centre (BDC) to top dead centre (TDC).

Process 2–3: it is a constant-volume heat transfer to the working gas from an external source while the piston is at top dead centre. This process is intended to represent the ignition of the fuel-air mixture and the subsequent rapid burning.

Process 3–4:it is an adiabatic (isentropic) expansion (power stroke).

Process 4–1:it is completes the cycle by a constant-volume process in which heat is rejected from the air while the piston is at bottom dead centre.

Process 1–0:it is the mass of air is released to the atmosphere in a constant pressure process.

The Otto cycle consists of isentropic compression, heat addition at constant volume, isentropic expansion, and rejection of heat at constant volume. In the case of a four-stroke Otto cycle, technically there are two additional processes: one for the exhaust of waste heat and combustion products at constant pressure (isobaric), and one for the intake of cool oxygen-rich air also at constant pressure; however, these are often omitted in a simplified analysis. Even though those two processes are critical to the functioning of a real engine, wherein the details of heat transfer and combustion chemistry are relevant, for the simplified

analysis of the thermodynamic cycle, it is more convenient to assume that all of the waste-heat is removed during a single volume change.

2.1.1.OTTO CYCLE ANALYSIS:

In processes 1–2 the piston does work on the gas and in process 3–4 the gas does work on the piston during those isentropic compression and expansion processes, respectively. Processes 2–3 and 4–1 are isochoric processes; heat transfer occurs but no work is done on the system or extracted from the system. No work is done during an isochoric (constant volume) process because addition or removal of work from a system as that requires movement of the boundaries of the system; hence, as the cylinder volume does not change, no shaft work is added or removed from the system.

Four different equations are used to describe those four processes. A simplification is made by assuming changes of the kinetic and potential energy that take place in the system (mass of gas) can be neglected and then applying the first law of thermodynamics (energy conservation) to the mass of gas as it changes state as characterized by the gas's temperature, pressure, and volume.[2][7]

During a complete cycle, the gas returns to its original state of temperature, pressure and volume, hence the net internal energy change of the system (gas) is zero. As a result, the energy (heat or work) added to the system must be offset by energy (heat or work) that leaves the system. In the analysis of thermodynamic systems, the convention is to account energy that enters the system as positive and energy that leaves the system is accounted as negative.

Equation 1a.

During a complete cycle, the change of energy of the system is zero:

The above states that the system (the mass of gas) returns to the original thermodynamic state it was in at the start of the cycle.

Where is energy added to the system from 1–2–3 and is energy is removed from 3–4–1. In terms of work and heat added to the system

Equation 1b:

Each term of the equation can be expressed in terms of the internal energy of the gas at each point in the process:

The energy balance Equation 1b becomes

If the internal energies are assigned values for points 1,2,3, and 4 of 1,5,9, and 4 respectively (these values are arbitrarily but rationally selected for the sake of illustration), the work and heat terms can be calculated.

The energy added to the system as work during the compression from 1 to 2 is

The energy added to the system as heat from point 2 to 3 is

The energy removed from the system as work during the expansion from 3 to 4 is

The energy removed from the system as heat from point 4 to 1 is

The energy balance is

Note that energy added to the system is counted as positive and energy leaving the system is counted as negative and the summation is zero as expected for a complete cycle that returns the system to its original state.

From the energy balance the work out of the system is:

The net energy out of the system as work is -1, meaning the system has produced one net unit of energy that leaves the system in the form of work.

The net heat out of the system is:

As energy added to the system as heat is positive. From the above it appears as if the system gained one unit of heat. This matches the energy produced by the system as work out of the system.

Thermal efficiency is the quotient of the net work from the system, to the heat added to system. Equation 2:

Alternatively, thermal efficiency can be derived by strictly heat added and heat rejected.

Supplying the fictitious values

In the Otto cycle, there is no heat transfer during the process 1–2 and 3–4 as they are isentropic processes. Heat is supplied only during the constant volume processes 2–3 and heat is rejected only during the constant volume processes 4–1.

The above values are absolute values that might, for instance, have units of joules (assuming the MKS system of units are to be used) and would be of use for a particular engine with particular dimensions. In the study of thermodynamic systems the extensive quantities such as energy, volume, or entropy (versus intensive quantities of temperature and pressure) are placed on a unit mass basis, and so too are the calculations, making those more general and therefore of more general use. Hence, each term involving an extensive quantity could be divided by the mass, giving the terms units of joules/kg (specific energy), meters3/kg (specific volume), or joules/(kelvin·kg) (specific entropy, heat capacity) etc. and would be represented using lower case letters, u, v, s, etc.

Equation 1 can now be related to the specific heat equation for constant volume. The specific heats are particularly useful for thermodynamic calculations involving the ideal gas model.

Rearranging yields:

Inserting the specific heat equation into the thermal efficiency equation (Equation 2) yields.

Upon rearrangement:

Next, noting from the diagrams (see isentropic relations for an ideal gas), thus both of these can be omitted. The equation then reduces to:

Equation 2:

Since the Otto cycle uses isentropic processes during the compression (process 1 to 2) and expansion (process 3 to 4) the isentropic equations of ideal gases and the constant pressure/volume relations can be used to yield Equations 3 & 4.[8]

Equation 3:

Equation 4:

where

is the specific heat ratio

The derivation of the previous equations are found by solving these four equations respectively (where is the specific gas constant):

Further simplifying Equation 4, where is the compression ratio :

Equation 5:

From inverting Equation 4 and inserting it into Equation 2 the final thermal efficiency can be expressed as.

Equation 6:

From analyzing equation 6 it is evident that the Otto cycle efficiency depends directly upon the compression ratio . Since the for air is 1.4, an increase in will produce an increase in

. However, the for combustion products of the fuel/air mixture is often taken at approximately 1.3. The foregoing discussion implies that it is more efficient to have a high compression ratio. The standard ratio is approximately 10:1 for typical automobiles. Usually this does not increase much because of the possibility of autoignition, or "knock", which places an upper limit on the compression ratio.[2] During the compression process 1–2 the temperature rises, therefore an increase in the compression ratio causes an increase in temperature. Autoignition occurs when the temperature of the fuel/air mixture becomes too high before it is ignited by the flame front. The compression stroke is intended to compress the products before the flame ignites the mixture. If the compression ratio is increased, the mixture may auto-ignite before the compression stroke is complete, leading to "engine knocking". This can damage engine components and will decrease the brake horsepower of the engine.

2.1.2 POWER:

The power produced by the Otto cycle is the energy developed per unit of time. The Otto engines are called four-stroke engines. The intake stroke and compression stroke require one rotation of the engine crankshaft.The power stroke and exhaust stroke require another rotation. For two rotations there is one work generating stroke.

From the above cycle analysis the net work produced by the system was:

(again,using the sign convention, the minus sign implies energy is leaving the system as work)

If the units used were MKS the cycle would have produced one joule of energy in the form of work. For an engine of a particular displacement, such as one liter, the mass of gas of the system can be calculated assuming the engine is operating at standard temperature (20 °C) and pressure (1 atm).Using the Universal Gas Law the mass of one liter of gas is at room temperature and sea level pressure:

V=0.001 m3, R=0.286 kJ/(kg·K), T=293 K, P=101.3 kN/m2

M=0.00121 kg

At an engine speed of 2000 RPM there is 1000 work-strokes/minute or 16.7 work-strokes/second.

Power is 16.7 times that since there are 16.7 work-strokes/second

If the engine is multi-cylinder, the result would be multiplied by that factor. If each cylinder is of a different liter displacement, the results would also be multiplied by that factor. These results are the product of the values of the internal energy that were assumed for the four states of the system at the end each of the four strokes (two rotations). They were selected only for the sake of illustration, and are obviously of low value. Substitution of actual values from an actual engine would produce results closer to that of the engine. Whose results would be higher than the actual engine as there are many simplifying assumptions made in the analysis that overlook inefficiencies. Such results would overestimate the power output.

2.1.3INCREASING POWER AND EFFICIENCY:

The difference between the exhaust and intake pressures and temperatures suggest that some increase in efficiency can be gained by removing from the exhaust flow some part of the remaining energy and transferring that to the intake flow to increase the intake pressure. A gas turbine can extract useful work energy from the exhaust stream and that can then be used to pressurize the intake air. The pressure and temperature of the exhausting gases would be reduced as they expand through the gas turbine and that work is then applied to the intake gas stream, increasing its pressure and temperature. The transfer of energy amounts to an efficiency improvement and the resulting power density of the engine is also improved. The intake air is typically cooled so as to reduce its volume as the work produced per stroke is a direct function of the amount of mass taken into the cylinder; denser air will produce more work per cycle. Practically speaking the intake air mass temperature must also be reduced to prevent premature ignition in a petrol fueled engine; hence, an inter-cooler is used to remove some energy as heat and so reduce the intake temperature. Such a scheme both increases the engine's efficiency and power density.

The application of a supercharger driven by the crankshaft does increase the power output (power density) but does not increase efficiency as it uses some of the net work produced by the engine to pressurize the intake air and fails to extract otherwise wasted energy associated with the flow of exhaust at high temperature and a pressure to the ambient

2.2 COMPRESSION RATIO:

With both air and fuel in a closed cylinder, compressing the mixture too much poses the danger of auto-ignition — or behaving like a diesel engine. Because of the difference in burn rates between the two different fuels, petrol engines are mechanically designed with different timing than diesels, so to auto-ignite a petrol engine causes the expansion of gas inside the cylinder to reach its greatest point before the cylinder has reached the "top dead center" (TDC) position. Spark plugs are typically set statically or at idle at a minimum of 10 degrees or so of crankshaft rotation before the piston reaches TDC, but at much higher values at higher engine speeds to allow time for the fuel-air charge to substantially complete combustion before too much expansion has occurred - gas expansion occurring with the piston moving down in the power stroke. Higher octane petrol burns slower, therefore it has a lower propensity to auto-ignite and its rate of expansion is lower. Thus, engines designed to run high-octane fuel exclusively can achieve higher compression ratios.

2.3.SPEED AND EFFICIENCY:

Petrol engines run at higher speeds than diesels, partially due to their lighter pistons, connecting rods and crankshaft (a design efficiency made possible by lower compression ratios) and due to petrol burning more quickly than diesel. Because pistons in petrol engines tend to have much shorter strokes than pistons in diesel engines, typically it takes less time for a piston in a petrol engine to complete its stroke than a piston in a diesel engine. However the lower compression ratios of petrol engines give petrol engines lower efficiency than diesel engines.

2.4.APPLICATIONS:

Petrol engines have many applications:

Automobiles Motorcycles Aircraft Motorboats Small engines, such as lawn mowers, chainsaws and portable engine-generators

2.5.DESIGN:

2.5.1WORKING CYCLES

4-Stroke Petrol engine.

Petrol engines may run on the four-stroke cycle or the two-stroke cycle. For details of working cycles see:

Four-stroke cycle Two-stroke cycle Wankel engine

2.5.2:CYLINDER ARRANGEMENT

Common cylinder arrangements are from 1 to 6 cylinders in-line or from 2 to 16 cylinders in V-formation. Flat engines – like a V design flattened out – are common in small airplanes and motorcycles and were a hallmark of Volkswagen automobiles into the 1990s. Flat 6s are still used in many modern Porsches, as well as Subarus. Many flat engines are air-cooled. Less common, but notable in vehicles designed for high speeds is the W formation, similar to having 2 V engines side by side. Alternatives include rotary and radial engines the latter typically have 7 or 9 cylinders in a single ring, or 10 or 14 cylinders in two rings.

2.5.3COOLING

Petrol engines may be air-cooled, with fins (to increase the surface area on the cylinders and cylinder head); or liquid-cooled, by a water jacket and radiator. The coolant was formerly water, but is now usually a mixture of water and either ethylene glycol or propylene glycol. These mixtures have lower freezing points and higher boiling points than pure water and also prevent corrosion, with modern antifreezes also containing lubricants and other additives to protect water pump seals and bearings. The cooling system is usually slightly pressurized to further raise the boiling point of the coolant.

2.5.4.IGNITION

Petrol engines use spark ignition and high voltage current for the spark may be provided by a magneto or an ignition coil. In modern car engines the ignition timing is managed by an electronic Engine Control Unit.

2.5.5.POWER MEASUREMENT

The most common way of engine rating is what is known as the brake power, measured at the flywheel, and given in kilowatts (metric) or horsepower (Imperial/USA). This is the actual mechanical power output of the engine in a usable and complete form. The term "brake" comes from the use of a brake in a dynamometer test to load the engine. For accuracy, it is important to understand what is meant by usable and complete. For example, for a car engine, apart from friction and thermodynamic losses inside the engine, power is absorbed by the water pump, alternator, and radiator fan, thus reducing the power available at the flywheel to move the car along. Power is also absorbed by the power steering pump and air conditioner (if fitted), but these are not installed for a power output test or calculation. Power output varies slightly according to the energy value of the fuel, the ambient air temperature and humidity, and the altitude. Therefore, there are agreed standards in the USA and Europe on the fuel to use when testing, and engines are rated at 25 ⁰C (Europe), and 64 ⁰F (USA)[citation needed] at sea level, 50% humidity. Marine engines, as supplied, usually have no radiator fan, and often no alternator. In such cases the quoted power rating does not allow for losses in the radiator fan and alternator. The SAE in USA, and the ISO in Europe publish standards on exact procedures, and how to apply corrections for deviating conditions like high altitude.

Car testers are most familiar with the chassis dynamometer or "rolling road" installed in many workshops. This measures drive wheel brake horsepower, which is generally 15-20% less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer. A YouTube video shows workshop measurement of a car's power. The measured power curve in kW is shown at 3:39.