汽车构造英文Chapter 9 Engine measurement and performanceWord格式.docx

1、Technical termsReview questionsManufacturers provide specification of their engines and also performance data which includes power output and torque. This chapter provides definitions of some of these basic terms as well as other information that relates to engines.There are a number of definitions

2、that relate to basic mechanics. Each of these has a bearing on the others. If force is considered first, then its meaning can be used to understand work. Work can be used to understand power, and so on.ForceA force exists whenever movement occurs or tends to occur. Forces are present everywhere. The

3、y start movement, stop movement and cause changes in direction. They are also present even when no movement occurs. The common terms push and pull are really pushing force and pulling force.A brief definition of force is “that which changes or tends to change the state of motion of a body”. Force is

4、 stated in newtons （N）.In Figure 9.1, pressure from combustion produces a force on the head of the piston and this is transmitted through the connecting rod to the crankshaft.Figure 9.1 Pressure from combustion produces a force on the head of the pistonWorkWork is done whenever the force applied to

5、an object causes it to move. Work is equal to the force applied multiplied by the distance moved. That is, the newtons （force） multiplied by metres （distance）. The result is newton metres （Nm）. One newton metre is given the name of joule, and so work is stated in joules （J）.For work to be done, move

6、ment must occur.Relating this to a piston in a cylinder of an engine, the work that is done as the piston moves down on its power stroke is equal to the average force on the piston multiplied by the piston stroke.The pressure on the piston that produces the force is referred to as the mean effective

7、 pressure. Mean, in this instance, has the meaning average, so mean effective pressure is the average combustion pressure on the piston during the power stroke.PowerPower is the rate （or speed at which work is done. This introduces the unit of time in addition to the unit of work （J）. Power is state

8、d in watts （ W ）.One watt is one joule （of work） performed in one second. If this is related back to the basic units, then one watt is equal to the force of one newton being moved through a distance of one metre in one second.A larger unit of power is the kilowatt （kW） （1000 W）. Engine power is stat

9、ed in kilowatts .HorsepowerAnother unit of power, which was used to indicate the power of engines, is horsepower. One horsepower is equal to 746 watts （units of power）. Its name comes from the power of a horse, and originated when comparisons were being made between the rate at which work could be d

10、one by the newly developed machines of the day, and by horses, which were then the source of power. Horsepower has been superseded by kilowatts.EnergyThis is the capacity or ability to do work. Energy is stated in the same units as work, that is, joules, as it can be classed as stored work. For exam

11、ple, energy is stored in a rotating flywheel. The flywheel absorbs energy during the power strokes and gives up energy during the other strokes.TorqueTorque is a twisting or turning force, which is equal to the force multiplied by the perpendicular distance to the turning point. Torque is stated in

12、newton metres （Nm）.In the cylinder of an engine, torque is produced by the force from the connecting rod applied 1o the crank of the crankshaft （Figure 9.2）. The diagram shows the crank at 90to the connecting rod. This would be the position of greatest torque if the force from the connecting rod was

13、 constant, but this is not so.Figure 9.2 Engine torque is equal to the force F multiplied by the perpendicular distance dThe torque change continuously as the crankshaft rotates because the force on the piston varies throughout its power stroke, and also because the angle between the connecting rod

14、and the crank x varies.The following are terms and definitions that are used in relation to engines.Bore and strokeThe size of an engine cylinder is referred to in terms of its bore and stroke （Figure 9.3）. The bore is the diameter of the cylinder. The stroke is the distance the piston travels from

15、TDC （top dead-centre） to BDC （bottom dead centre）.The bore is usually mentioned first. For example, the cylinder could have a diameter, or bore. of 80 mm and a stroke of 100mm. These measurements are used to determine the piston displacement.Engines can have small bores and long strokes, or large bo

16、res and short strokes. If an engine has a bore the same dimension as its stroke, it would be referred to as a square engine. If the diameter of the bore is larger than the stroke, then it is referred to as an over square engine.Piston displacementPiston displacement is the volume that the piston dis

17、places as it moves from BDC to TDC. Piston displacement of the cylinder in Figure 9.3 （as an example） would be the volume of a cylinder that has a 4 cm radius and is 10 cm high.The volume is:If the engine has four cylinders, the total displacement would be 2012.Engine capacityThe total piston displa

18、cement is also referred to as the capacity of the engine. The capacity is stated in litres, so an engine with 2012 cm displacement has a capacity of 2.012 litres. This would be rounded off and the engine would be classed as a 2 litre engine.Litres are used for engine capacity in the International Sy

19、stem of Units （SI） because it is the unit for fluids. Fluids are both liquids and gases and it is the gas capacity of the engine cylinders that is being measured. Older engines that have their bore and stroke measured in inches have their capacity stated in cubic inches.Compression ratioThe compress

20、ion ratio of an engine is a measurement of how much the air-fuel charge is compressed in the cylinder, It is calculated by dividing the volume of the cylinder （and its combustion chamber） with the piston at BDC, by the volume with the piston at TDC. This is shown for a simple cylinder in Figure 9.4.

21、Figure 9.4 The compression ratio is the volume in a cylinder with the piston at BDC divided by its volume with the piston at TDC The volume with the piston at TDC is called the clearance volume, since it is the clearance that remains above the piston when it is at TDC.ExampleA cylinder has a total v

22、olume of 600 cm and a clearance volume of 75 cm. The compression ratio is 600 divided by 75cm -that is 8:1. During the compression stroke, the air-fuel mixture is compressed from a volume of 600 cm to 75 cm, or to one-eighth of its original volume.The volume of the actual piston displacement is ofte

23、n referred to as the swept volume, since this is the volume of air that is swept into the cylinder by the action of the piston as it moves down its stroke. The total cylinder volume is then equal to the swept volume plus the clearance volume. The compression ratio can then be stated as:（swept volume

24、 + clearance volume）/（clearance volume）This is the same as the total volume divided by the clearance volume.A compression ratio of around 8:1 would be used for a petrol engine. High compression ratios provide high pressure of the air-fuel mixture. As a result, higher combustion pressures are obtaine

25、d and more force is exerted on the piston during the power stroke.However, there is a limit to the compression ratio of petrol engines. If the air-fuel mixture is compressed too much, detonation and rough running will occur.Diesel engines have compression ratios from about 15:1 to 21:1. High compres

26、sion ratios such as these are needed to obtain the high an temperatures for compression ignition.The power output of an engine is measured in kilowatts （kW）. If an engine is rated at 100 kW, this means that the engine can produce this power at a particular engine rpm at full throttle. Manufacturers

27、rate engines at the maximum kilowatts that can be produced.To measure power, the engine is coupled to a dynamometer in a testing laboratory （ Figure 9.5 ）. This is a braking device, which can be in the form of a water pump or an electric dynamo. The dynamometer applies a load and also enables the lo

28、ad to be varied. Instruments are arranged to measure the engines rpm and the load placed on the engine. This enables the power output in kW to be determined.Figure 9.5 An engine on a test bed with an engine dynamometer being used to check engine output TOYOTAThe dynamometer used for service as work

29、checks the engine in the vehicle. This type of unit is called a chassis dynamometer （Figure 9.6）. The driving wheels of the car are placed on roller; the engine drives the wheels and the wheels drive the rollers. The rollers can be loaded and the load varied so that output can be measured.Figure 9.6

30、 Front-wheel drive vehicle on the rollers of a chassis dynamometer MAZDAThe power delivered by the wheels to the rollers is shown on a gauge as road power. This is a practical term because this is the power that is available at the wheels to move the vehicle. Due to friction losses in the drive line

31、, road power will be less than the power output of the engine.Indicated power （power input）Indicated power is based on the power developed inside the engine cy1inders by the combustion processes. This is the power input of the engine.To measure the power input （indicated power） a special indicating device （an oscilloscope） is required. This device, which is used in a laboratory, measures the pressure in the cylinder throughout the four piston strokes （intake, compression, power, exhaust ）.A graph of the cylinder pressures for a petrol engine is shown in Figure 9.7.