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Testing and performance of IC engines

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Testing and performance of IC engines

With a growing demand for transportation IC engines have gained lot of importance in automobile industry. It is therefore necessary to produce efficient and economical engines. While developing an IC engine it is required to take in consideration all the parameters affecting the engines design and performance. There are enormous parameters so it becomes difficult to account them while designing an engine. So it becomes necessary to conduct tests on the engine and determine the measures to be taken to improve the engine's performance.

Contents

1. Power and Mechanical Efficiency

2. Fuel Air Ratio

3. Volumetric Efficiency

4. Specific Output

5. Specific Fuel Consumption

6. Thermal Efficiency and Heat Balance

7. Exhaust Smoke and Emissions

8. Effective Pressure and Torque

Power and mechanical efficiency

An IC engine is used to produce mechanical power by combustion of fuel. Power is referred to as the rate at which work is done. Power is expressed as the product of force and linear velocity or product of torque and angular velocity. In order to measure power one needs to measure torque or force and speed. The force or torque is measured by Dynamometer and speed by Tachometer. The power developed by an engine and measured at the output shaft is called the brake power (bp) and is given by,

b p = 2 π N τ 60

where:

τ is the torque, in Newton meter (N.m), N is the rotational speed, in minutes, b p is the brake power, in watt.

However while calculating the Mechanical efficiency another factor called Indicated Power (ip) is considered. It is defined as the power developed by combustion of fuel in the engine cylinder. It is always more than brake power and is given by,

i p = P V N K 60

where:

p is the mean pressure, V is the displacement volume of the piston k is the number of cylinders

Therefore, the difference between ip and bp indicates the power loss in the mechanical components of engine (due to friction). So the mechanical efficiency is defined as ratio of brake power to the indicated power.

E = b p i p or  : E = b p b p + f p

Friction power is the difference between indicated power and brake power.

f p = i p b p

Air–fuel ratio

It is the ratio of mass of fuel to mass or volume of air in mixture. It affects the phenomenon of combustion and used for determining flame propagation velocity, the heat released in combustion chamber. For practise always relative air fuel ratio is defined. It is the ratio of actual air–fuel ratio to that of the stoichiometric air fuel ratio required for burning of fuel which is supplied.

Equivalence ratio,Ø : ( F / A ) = A c t u a l f u e l a i r r a t i o / S t o i c h i o m e t r i c f u e l a i r r a t i o

Relative ratio,λ : ( A / F ) = A c t u a l a i r f u e l r a t i o / S t o i c h i o m e t r i c a i r f u e l r a t i o

Thermal efficiency and heat balance

It is the ratio of output to that of energy input in the form of fuel. It gives the efficiency with which the chemical energy of fuel is converted into mechanical work. It shows that all chemical energy of fuel is not converted into heat energy.

Thermal efficiency and total energy input- The methodology for calculating thermal efficiency of a unit is described in this section to help to determine whether the unit qualifies to exemption or not. It also includes total energy input which also helps in determining thermal efficiency.

The thermal efficiency standard for a unit are decided by CAIR model trading rules, the CAIR FIP, CAMR, the CAMR Hg model trading rule, and the proposed CAMR Federal Plan, EPA. However, the thermal efficiency standard applicable to all fuels combusted by a unit are decided by the United States Environmental Protection Agency (EPA), while the application of the standard to natural gas and oil are made by FERC.

Brake specific fuel consumption (BSFC)

It is defined as the amount of fuel consumed for each unit of brake power per hour; it indicates the efficiency with which the engine develops the power from fuel. It is used to compare performance of different engines.

The amount of fuel which an engine consumes is rated by its brake specific fuel consumption (BSFC). In the U.S. this is generally computed in dimensions of pounds of fuel per horsepower per hour. For most internal combustion engines the BSFC will be in the range of 0.5 to 0.6. This estimate fits well with the old rule of thumb that an engine will burn HP/10 gallons per hour. Here is an example with a 90 HP engine. When developing 90 HP, how much fuel will it burn? Assume a BSFC of 0.55 and gasoline at 6.25 lbs/gallon:

  • 90 x 0.55 = 49.5 pounds of fuel burned per hour
  • 49.5 x 1/6.25 = 7.92 gallons per hour
  • The fuel efficiency will tend to peak at higher engine speeds. At near wide-open throttle the BSFC will be closer to a value of 0.5. The BSFC tends to be the same for similar engines. Really huge diesel engines have reported BSFC values in the 0.35 range. The estimate of brake specific fuel consumption for two-stroke engines ranges from 0.55 to as high as 0.8 pounds of fuel per horsepower per hour.

    Measurement of brake power

    The torque and the angular speed measurement of engine are involved in measurement of brake power. Dynamometer is used for torque measurement. The rotor of the engine which is under state is connected to rotor. Rotor moves through distance 2πr against force F. Hence work done,

    W = 2 π r F

    They are of two types-

    Absorption dynamometer

    It absorbs and measures output power of engine. This power is dissipated in the form of heat. e.g., prony brake, hydraulic dynamometer, rope dynamometer, eddy current dynamometer, swinging field d.c. dynamometer etc. Absorption dynamometers are ideally suited for testing petrol engines for mopeds and electrical F.H.P. motors. Their main advantage lies in the fact that they are self-air-cooled and hence water cooling or additional air cooling is not required. This advantage is particularly significant in case of moped engines and F.H.P. motors, which are also air-cooled.

    Transmission dynamometer

    In this the power is transmitted to load connected to engine. Torque meter is alternative name of this dynamometer. It is usually consist of strain gauge which measures the torque by angular deformation of shaft. These dynamometers are accurate and widely used in automatic units. It is available in both electric motor and hydrostatically driven versions, the AIDCO Model 450E and 450 Transmission Dynamometer Test Stands are designed for automatic transmission dyno testing and for testing power shift transmissions used in commercial applications built by Allison, ZF, Voith, and Renk, as well as most military transmissions.

    Exhaust smoke and other emission

    Smoke and other emission are undesirable for public environment. Because of global warming and emphasis on air pollution all possible things are tried to keep them low. Smoke also indicates incomplete combustion of fuel.

    Here are some tips of what you can adopt as air pollution solutions:

  • Air conditioning systems and electrical gadgets within the vehicle (e.g. sound system, mobile tv systems) also take up energy. So if they are not in use, turn them off.
  • If you are waiting while in your car, turn off the car engine. Idling actually wastes your fuel, and money, and also contributes to pollution (e.g. exhaust air, noise pollution etc). So reduce the amount of idling as much as possible, whether it is when you are waiting for someone while in the car, or waiting to find a carpark lot, or waiting for your turn in a drive-through queue. These days, it seems that there are new car designs where the engines stop automatically when the car is stationary, but the engine restarts very quickly when you need to move off. Such green technology can help you save fuel, money and reduce pollution, so do look out for these models.
  • Keep your car in efficient working condition. This is one of the important air pollution solutions for cars, because poorly maintained cars are a major source of car pollution. Not only will this help you reduce your repair costs, it will also help you to reduce the amount of pollution your car creates.
  • Also, check the pressure of your car tires regularly. Tires that are properly inflated can actually help you reduce your fuel consumption (and corresponding pollution) by up to 3%, whereas tires with low pressures actually cause drag and increase fuel consumption, which in turn brings about more pollution.
  • Get rid of excess load in your car, for example, the cargo that you have been carrying in your boot for months because you keep forgetting to bring the stuff home. According to WikiHow, every extra 100 pound in your car gives you up to 2% less mileage. This also means more pollution for the same distance travelled.
  • Mean effective pressure and torque

    Mean effective pressure is an important parameter for comparing the performance of different engines. It is defined as the average pressure acting over piston throughout a power stroke. It is given by the following relation,

    p = i p 60 L A R K

    where:

    p is the Mean Effective Pressure, i p is Indicated Power

    e Indicated power watt

    A is the Area of the Piston R is the Rotational Speed k is the Number of Cylinders, L is stroke length

    If mean effective pressure is based on brake power(bp) then it is referred to as brake mean effective pressure(bmep). If it is based on indicated power(ip) it is called indicated mean effective pressure(imep). friction mean effective power is the difference of imep and bmep,

    f m e p = i m e p b m e p

    Mean effective pressure also has an effect on torque. Torque could be expressed by following relation also

    τ = b m e p A R K 2 π

    Mean effective pressure and torque both are affected by the size of engine. A large engine produces more Torque for the same mean effective pressure. For this reason engines mean effective pressure gives indication of its displacement utilization and not torque. Power of an engine is dependent on its size so it is not possible to compare different engines based on their power or torque. Therefore, mean effective pressure is the true indication of the relative performance of different engines.

    Volumetric efficiency

    It is the ratio of the actual volume of the charge drawn in during the suction stroke to the swept volume of the piston. The amount of air taken inside the cylinder is dependent on the volumetric efficiency of an engine and hence puts a limit on the amount of fuel which can be efficiently burned and the power output. The value of volumetric efficiency of a normal engine lies between 70 and 80 percent, but for engines with forced induction it may be more than 100 percent. Naturally aspirated engines can have volumetric efficiencies of more than 100% by using properly designed induction piping, utilizing resonance in the induction pipe (by selecting the induction pipe length according to the rotation speed at which maximum VE is desired) as well as the inertia of the air mass in the induction piping. Using inertia effects requires high air speeds in the induction system, which is normally accompanied by high flow losses. By careful design and streamlining of the inlet port and valves, much of the losses can be reduced to an acceptable level. Resonance and inertia effects are normally only used in high speed sports engines, for example as found in many modern motorcycles.

    References

    Testing and performance of IC engines Wikipedia


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