LOAD TEST ON A PERKINS DIESEL ENGINE

Instructed by :

Introduction

Name :

Index No :

Practical No : 05

Course : B.Sc Engineering

The diesel engine which was invented by Rudolph Diesel has a long history

that is intertwined nearly with economic and other issues of that time. He could develop the

diesel engine by thinking of how it operates and principles of operation. He thought up the concept of the engine that compresses air to the degree where there is a resulting rise in

temperature. The concept followed the principle where when the air enters the chamber with the piston, the air ignited due to the high temperatures. This causes the piston to move down and eliminates the need for an ignition source. When Diesel designed his engine, it was in a time when there was a demand for a more fuel efficient engine as the steam engine was no where close to efficient.

The principle behind the diesel engine is not as simple as it indicates. The conversion into the practice was very problematic. Such high pressures and temperatures had never been used before, and the first experimental engine, built 1893 together with the Maschinenfabrik Augsburg (MAN) in Germany led to its destruction. Only a second engine, built 1896, could convince the engineers and performed an efficiency of about 25 percent, which was by far more than any other engine's performance at that time. But the engine was not after Diesel's requires yet: The compression ratio was still low and the max. pressure therefore small (about 30 bar), additionally a fuel injection was not yet possible. He had to use an air-injection, a procedure, which required many very complicated, expensive and heavy additional devices. This engine could become generally accepted only with many difficulties, because of economic problems - fuel oil and petroleum were very expensive - and disputes about patents delayed a successful introduction.

Diesel Engine built in February of 1897. Source: Helmut Hütten, "Motoren", Motorbuchverlag Stuttgart, S. 19

The diesel internal combustion engine differs from the gasoline powered Otto cycle by using highly compressed, hot air to ignite the fuel rather than using a spark plug. In the true diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 22:1 resulting in 40-bar (4.0 Mpa) pressure compared to 8 to 14 bars (0.80 to 1.4 MPa) in the petrol engine. This high compression heats the air to 550 °C (1,022 °F). At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber. The start of aporisation causes a delay period during ignition, and the characteristic diesel knocking sound as the vapor reaches ignition temperature and causes an abrupt increase in pressure above the piston. The rapid expansion of combustion gases then drives the piston downward, supplying power to the crankshaft. Model aero plane engines use a variant of the Diesel principle but premix fuel and air via a carburetion system external to the combustion chambers.

Procedure

Open the inlet valve fully and the outer valve slightly (of the dynamometer).

Make sure all the valves in the piping between the source of the water supply and the dynamometer inlet are fully open.

The engine may now be started.

Loads may be regulated by opening the sluice gates by means of the hand-wheel and simultaneously opening the engine throttle, until the desired load and speed obtained.

Adjust the outlet valve to pass sufficient water to keep the temperature at a reasonable figure.

A hand-wheel is provided on top of the balance frame to adjust the height of the balance arm. Make sure that this wheel is always set to the horizontal position when taking B.H.P. readings.

Calculations

The brake power of the engine is

Here K is a constant and value of it equal to 4500 K = 4500. W is load in pounds and N is speed in rpm.

I.P = B.P + F.P

Specific fuel consumption (SFC) = (Fuel consumption/Brake power)*3600 kg/kWh.

Brake mean effective pressure (BMEP) = (Brake power)/ ( )

Volumetric efficiency = (Volume inhaled/swept volume).

Mechanical efficiency =B.P /I.P .

Brake thermal efficiency = overall efficiency.

Indicated thermal efficiency =I.P /Rate of heat input.

Volume Inhaled =

Swept volume = 4*

Overall Efficiency = B.P/Rate of heat input by the fuel.

Given data: 4 stroke 4 cylinder engine, Calorific value of fuel = 44290kJ/kg, Relative density of diesel = 0.88

Cp for exhaust gas is 1.08 kJ/kgK

Trial 1 2 3 4

Break power (kW) 0 3 5.333333333 9

BEMP (kNm) 0 138.8165243 246.7849321 416.449573

volume inhaled (m3/min) 0.01274567 0.01274567 0.01274567 0.01274567

volumetric Efficiency 0.009829498 0.009829498 0.009829498 0.009829498

FUEL CONSUMPTION ( kg/s) 0.000349206 0.000463158 0.000571429 0.00999198

specific fuel consumption (SFC) kg/kWh 0 0.555789474 0.385714286 0.279365079

friction power F.P.(kW)calculated using the graph

8.9743 8.9743 8.9743 8.9743

indicated power IP(kW)

8.9743 11.9743 14.3076 17.9743

over all efficiency (break thermal efficiency)

Indicate thermal efficiency

HEAT BALANCING CHAT

heat input by the fuel kJ/s

heat taken out by water kJ/s

heat taken out by Air kJ/s

Break power (kW)

indicated powe IP

Calculation

Trial 1 2 3 4

Break power (kW) 0 3 5.333333333 9

BEMP (kNm) 0 138.8165243 246.7849321 416.449573

volume inhaled (m3/min) 0.01274567 0.01274567 0.01274567 0.01274567

volumetric Efficiency 0.009829498 0.009829498 0.009829498 0.009829498

FUEL CONSUMPTION ( kg/s) 0.000349206 0.000463158 0.000571429 0.000698413

specific fuel consumption (SFC) kg/kWh 0 0.555789474 0.385714286 0.279365079

Friction power F.P.(kW)using the graph

8.9743 8.9743 8.9743 8.9743

indicated power IP(kW)

8.9743 11.9743 14.3076 17.9743

over all efficiency (break thermal efficiency)

0 0.1341 0.2079 0.2608

Indicate thermal efficiency

-0.4542 -0.2249 -0.0704 0.0373

HEAT BALANCING CHAT

heat input by the fuel kJ/s

heat taken out by water kJ/s

heat taken out by Air kJ/s

Break power (kW)

indicated powe IP

Discussion

Now we are going to have a look on the advantages of diesel engines over other internal combustion engines.

They burn less fuel than a petrol engine performing the same work, due to the engine's higher temperature of combustion and greater expansion ratio.  Gasoline engines are typically 25 percent efficient while diesel engines can convert over 30 percent of the fuel energy into mechanical energy.

They have no high-tension electrical ignition system to attend to, resulting in high reliability and easy adaptation to damp environments. The absence of coils, spark plug wires, etc., also eliminates a source of radio frequency emissions which can interfere with navigation and communication equipment, which is especially important in marine and aircraft applications.

The life of a diesel engine is generally about twice as long as that of petrol engine due to the increased strength of parts used, also because diesel fuel has better lubrication properties than petrol.

Diesel engines produce very little carbon monoxide as they burn the fuel in excess air except at full load, at which point a full stochiometric quantity of fuel is injected per cycle. However, they can produce black soot from their exhaust, consisting of unburned carbon compounds. This is often caused by worn injectors, which do not

atomize the fuel sufficiently, or a faulty engine management system which allows more fuel to be injected than can be burned with the available air. Other problems associated with the exhaust gases (nitrogen oxides, sulfur oxides) can be mitigated with further investment and equipment; some diesel cars now have catalytic converters in the exhaust.

For any given partial load the fuel efficiency (mass burned per energy produced) of a diesel engine remains nearly constant, as opposed to petrol and turbine engines which use proportionally more fuel with partial power outputs.

With a diesel, boost pressure is limited only by the strength of the engine components, not predestinations of the fuel charge as in petrol engines.

The basic disadvantage of diesel engine is that it is expensive. It expensive both in manufacturing (due to high work load) and also in maintenance. It is expensive due to ecological incompatibility of its exhaust and due to necessity to adjust its exhaust according to strict requirements of international agreements.

Moreover, due to the greater compression force required and the increased weight of the stronger components, starting a diesel engine is a harder task. More torque is required to push the engine through compression.

Compression ratio of a diesel engine.

In an auto-ignition diesel engine, (no electrical sparking plug- the hot air of compression lights the injected fuel) the Compression Ratio will customarily exceed 14:1. Ratios over 22:1 are common. The appropriate compression ratio depends on the design of the cylinder head. The figure is usually between 14:1 and 16:1 for direct injection engines and between 18:1 and 23:1 for indirect injection engines.