Energy Balance For An Internal Combustion Engine Engineering Essay

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The main objective of the experiment is to measure the energy contributions to the diesel engine, which is treated as a thermodynamics system. The energy contributions that are not measured may then be estimated from an energy balance.

There are two main, unmeasured energy contributions to identify:-

An energy loss from incomplete combustion, in which some of the fuel is not burned completely.

An energy loss by heat transfer to the air surrounding hot engine components.

Introduction

A Petter four stroke diesel engine will be used to investigate the efficiency of a diesel engine. Diesel engines are internal combustion engines designed to convert the chemical energy available in the fuel, into mechanical energy. This mechanical energy moves the pistons up and down inside cylinders. The pistons are connected to a crankshaft, and the up-and-down motion of the pistons, known as linear motion, creates the rotary motion needed to turn the wheels of a car forward.

We are already aware of the fact that internal combustion engines have very low efficiencies, but the purpose of such experiment is to study where and how the energy is used and lost. This would help us to improve the efficiency of the internal combustion engines where ever possible.

Theory

Both diesel engines and petrol engines convert fuel into energy through a series of small explosions or combustions. The major difference between diesel and petrol engines is the way these explosions happen. In a petrol engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engine, however, the air is compressed first, and then the fuel is injected, because as the air is compressed it heats up to around 400°C, it is hot enough to ignite fuel.

A four stroke diesel engine uses the following cycle (illustrated in Figure1):

Intake stroke -- The intake valve opens, and fresh air (containing no fuel), is drawn into the cylinder, moving the piston down.

Compression stroke -- As the piston rises, the air is compressed, causing its temperature to rise. At the end of the compression stroke, the air is hot enough to ignite fuel.

Combustion stroke -- As the piston reaches the top, fuel is injected at just the right moment and ignited, forcing the piston back down.

Exhaust stroke -- The piston moves back to the top, pushing out the exhaust created from the combustion out of the exhaust valve.

Figure 1

24075-004-613c6f14.gif

Eq1From the basic steady flow energy equation, using the air-cycle method, we may write:-

The value of can be approximated, closely, to:

, Cpe is taken as 1100J/KgK

It is convenient to replace by in order to make an allowance for the possibility of incomplete combustion.

*Where, FL, is the proportion of fuel energy that is not available because of incomplete combustion.

Heat transfer, (Qr), is the sum of energy transferred to cooling water and energy lost to the surrounding from hot engine components, therefore we can write:

Qr = -\dot mw Cpw ( Tout - Tin ) + Qrn

Replacing all quantities in Eq1 we get:

Symbols explained as follows:

rate of heat energy transfer of system

rate of the work done by the system (power)

combustion air mass flow rate

fuel mass rate

specific enthalpy of products of combustion

specific enthalpy of combustion air

specific heat of water 4190 J/kgK

Apparatus

The Apparatus used in this experiment is mentioned as follows:

Petter Diesel Engine: A four stroke, single cylinder, 659CC Petter Diesel Engine was used to conduct the experiment. Figure 2 shows the picture of Petter Diesel Engine used in the experiment.

Ear Muffles: Ear muffles were used to protect the ears from the loud noise of the diesel engine; prolonged exposure to such loud noise without ear protection can lead to hearing impairment.

Barometer: A Barometer was used to measure the atmospheric pressure at the time of experiment. Atmospheric pressure was needed to calculate the mass flow rate. The Barometer gives readings in mmHg. Figure 3 shows the picture of barometer used in the experiment.

IMG_4763.JPGIMG_4767.JPG

Figure 2 Figure 3

Experimental Procedure

The purpose of this experiment was to investigate the efficiency of a diesel engine. To start with the experiment, all the gauges on the apparatus were pre set to default readings and as a safety precaution all students were provided with ear muffles. Each individual group member was assigned a task by the module lecturer. My assigned task was to measure the oil and at the same time, time the engine as it consumed the set amount of diesel. Similarly other students were given tasks, which they carried on doing as the experiment progressed.

The engine was started and after waiting for the recommended time of ten minutes, all the readings were taken off gauges. A weight of 5kg was already placed onto the torque arm before starting the engine. Measurements such as fuel flow rate, cooling water flow-rate, spring balance, orifice plate pressure drop, speed gauge and electric thermometer were taken.

Electric thermometer reading is divided into four parts, mentioned as follows:

Exhaust temperature

Cooling water inlet temperature

Cooling water outlet temperature

Air inlet temperature

After taking all the readings, engine was shut down and ear muffles were removed. The readings taken off the gauges were then used to work out the energy balance for an internal combustion engine.

Results

MEASURED PROPERTY

VALUE

UNITS

Atmospheric Pressure

764

mmHg

Engine Speed

1500

rpm

Spring balance reading

15

N

Mass on torque arm

5

Kg

Amount of Fuel measured

20

ml

Time to consume fuel

97

Seconds

Relative density of fuel

0.864

-

Orifice plate pressure drop

31

mmH2O

Exhaust gas temperature

260

Degrees Celsius

Cooling water inlet temperature

69

Degrees Celsius

Cooling water outlet Temperature

75

Degrees Celsius

Air inlet Temperature

18.5

Degrees Celsius

Cooling water flow-rate

5

litres/minute

Following readings were obtained from the gauges:

1. Shaft power output= torque* shaft rotational speed

= W(kg load -dial reading)*rt*N(rpm)*2π/60

= (5*9.81)-15*0.4*1500*2π/60

= 2139.42 Watts

= 6.13 10-3 kg/s

3. Fuel flow rate =

= (20 10-3/1000) (864)

= Kg/s

4. Heat transfer rate to the cooling water = {(l/min)/60} * 4.196* (Tout - Tin)

= *4.196*(75-69)

= 2.098 kJ/s

5. Heat transfer to exhaust gases

= (6.24

= 1.70 kJ/s

6. Energy Balance:

Fuel Energy Input = mf*LCV

= (1.78x10-4) x (43x106)

= +7654W

Shaft Power Output= + 2139.42W

Cooling Water Heat Transfer= +2098W

Exhaust Heat Transfer = +1700W

Energy Transfer =Qm-mf *FL

= -7654+2139.42+2098+1700

= -1716.58W

Efficiency = n=useful work output

fuel energy input

=2139.42 x 100

7654

=27.95% (Useful Work)

Energy to surroundings = (Heat supplied in fuel - Useful work done - Energy to coolant - Energy to exhaust)

= 7.66 - 2.14 - 2.10 - 1.68

= 1.74KW

Percentage energy to coolant = Energy to coolant - 100

Heat supplied in fuel

= x 100

= 27.41%

Percentage to exhaust = Energy to exhaust - 100

Heat supplied in fuel

= x 100

= 21.93%

Percentage loss to the surrounding = Energy to surroundings - 100

Heat supplied in fuel

= x 100

= 22.71%

pie chart.jpg

Figure 5

Figure 4

Discussion

Engine efficiency refers to an engine's ability to transform the available energy from its fuel into useful work. The modern petrol combustion engine operates at an average of roughly 20 to 30 percent engine efficiency. The remaining 70 to 80 percent of the energy is lost to the surroundings in form of exhaust heat, mechanical sound energy and friction.

Diesel engines are a bit more efficient. The diesel engine uses high compression to ignite its fuel. This higher compression compensates for the engines heat losses and results in roughly 40 percent engine efficiency. This engine efficiency is only observed by direct injection diesel engines (discussed later). Rest 60 percent energy, like the petrol engine is lost to the surroundings.

The Petter diesel engine which was the subject of this experiment showed a poor overall efficiency compared to an average diesel engine. The overall efficiency of the Petter diesel engine was only 27.95 percent. The reason for such poor efficiency was that most of the energy was lost to surrounding, but that doesn't mean that the engine cannot me made more efficient.

See Figure 4 and 5 for the distribution of energy lost and utilised.

There are many ways to improve the efficiency of a diesel engine, some are discussed below:

Turbochargers: The purpose of a turbocharger is to compress the air flowing into the diesel engine, this lets the engine squeeze more air into a cylinder and more air means that more fuel can be added; basically a turbocharger converts waste energy from an engine's exhaust gases into compressed air, which it pushes into the engine. This allows the engine to burn more fuel producing more power and improves the overall efficiency of the combustion process, hence making the engine more efficient. However, the only disadvantage of a turbocharger is that, if it is in a car's engine it would take a few second to respond as the driver presses the pedal, this phenomenon is known as lag. Turbocharger suffers from lag because it takes a few moments before the exhaust gases reach a velocity that is sufficient to drive the impeller/turbine.

Direct Injection: With direct injection, the diesel fuel is directly injected into the cylinder i.e. fuel is mixed with air inside the cylinder, allowing for better control over the amount of fuel used, and varies depending on demand. This makes the engine more fuel efficient. Before direct injection, the fuel was mixed with air in the car's intake manifold.

Variable Valve Timing: Valves open and close to allow air and fuel to enter cylinders and for the products of combustion to exit. Different valve timings produce different results (more power and or better fuel economy). Many modern engines can vary valve timing, allowing the default low RPM range of the engine to have more economical timing, and the higher RPM range to go for max power.

Cylinder Deactivation: Internal Combustion Engines, with this feature can simply deactivate some cylinders when less power is required, temporarily reducing the total volume of the engine cylinders and so burning less fuel. This feature is mostly found on V6 and V8 engines.

Super Charger: Superchargers increase the intake of air into the combustion chamber. This means, more air into the combustion chamber and with more air, more fuel can be added, and more fuel means a bigger explosion and greater horsepower. Adding a supercharger is a good way to increase the power of a normal-sized engine and thus making it more efficient. Superchargers also create better fuel efficiency by increasing the amount of oxygen available to an engine's combustion chambers, because superchargers increase the power of the engine by utilising more oxygen, they do not require a larger engine and therefore also increase fuel efficiency by allowing cars to be lighter. The biggest disadvantage of superchargers however, is that they steal some of the engine's horsepower. A supercharger can consume as much as 20 percent of an engine's total power output but also generates as much as 46 percent additional horsepower. Since, it generates more power than it requires, it is generally thought to be good option to increase engines efficiency.

Conclusion

Diesel engines are a form of Internal Combustion Engines. They are very inefficient if working on their own. Generally about 25-30 percent energy is used in work and the rest is lost to surroundings. The efficiency of a diesel engine can be enhanced by at least 10-15 percent if combined with the modern technological devices, such as turbochargers and superchargers. Other clever techniques can also be used to improve the fuel efficiency and overall engine efficiency of the engine, such as direct injection, variable valve timing and cylinder deactivation.

Unfortunately, from the very beginning the focus on an internal combustion engine has been on producing more power rather than providing a better fuel economy, but nowadays due to growing awareness of environment and rising oil prices, engineers have shifted their attention on to producing more fuel efficient engines. For example, BMW is researching on ways to increase the fuel efficiency of a conventional engine by 10-15 percent by looking to recover and reuse heat energy lost through the exhaust and that absorbed by the engine cooling system. With such initiatives we can see a future of more efficient and more environmentally friendly engines.

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