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IC Engine Efficiency

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I.    Objectives:

To understand the fuel conversion efficiency an engine and to measure the fuel conversion efficiency of an 8-cylinder internal combustion engine. To compare the efficiency at wide open throttle with the efficiencies observed for loads similar to those required to travel at 60 miles per hour on a level highway or 30 miles per hour on a city street.  To observe the impact of throttle setting on the efficiency of the engine and to relate the pumping work required to operate the engine to the efficiency.  To compare peak pressure in cylinder 4 when the engine is producing around 120 HP to the pressure when it is producing 7 HP.

II.  Apparatus:

1.  An eight cylinder, fuel-injected, spark ignition, Ford engine with 4.6 liter (approximate) displacement and 32 valves.  The engine is computer controlled with the EEC-IV model controller.  This engine is used in the Lincoln Mark VIII and later in the Mustang Cobra.  For those interested in the details, here are the specifications of the engine.  Nominal engine ratings are: Torque 284 ft·lbf at 4,000 RPM.  Power 216 HP at 4000 RPM.

2. The various outputs of the EEK-IV are communicated by the Data Communications Link (DCL) and displayed on a dedicated computer.

3.  SuperFlow SF%u20111 water brake dynamometer and control module, SF-1809.

4. SuperFlow Data/Control System (Handheld Controller SF-1853, Sensor Box in Engine room, PC running SuperFlow WinDyn Software) including the transducers for measurement of engine speed, fuel flow, and air flow.

5. Kistler model 6127 quartz pressure transducer installed in cylinder 4. PCB Model 003A05 cable, model 402A02 charge amplifier and power supply. 

6. Shaft angle encoder reporting the angle of the crankshaft every 0.2°. 

III. Background:

The Engine

The internal combustion (IC) engine plays an important role in the industrialized world and the lives of its citizens.  The engine provides cheap, mobile power and has drastically changed the lives of ordinary people over the last seventy five years.  The IC engine is also a source of pollution, noise and CO2, which is a green house gas.  These facts, as well as the physical and political limitations on petroleum resources, insure that the IC engine will remain a source of controversy in industrialized society.  It is certain that engineering efforts to improve the characteristics of the IC engine will continue for some time to come.  Despite recent initiatives to build hybrid and fuel cell automobiles, the conventional IC engine continues to dominate the automotive industry.

The engine used here utilizes a 4 stroke operating cycle and is naturally aspirated, which means that air is pulled into the cylinders by the action of the pistons.  The engine is fuel injected which means that fuel is sprayed into the cylinder with injector pumps which can deliver controlled amounts of fuel at controlled times.  The air-fuel mixture is ignited with a spark from a spark plug and the resulting explosion drives the piston in the cylinder.  The crankshaft converts the resulting linear motion into circular motion.

Many engine functions are controlled by a computer referred to as an engine control module (EEC-IV).  The driver sets the position of the accelerator pedal which controls the position of the throttle plate.  Sensors inform the EEC-IV of the throttle position, engine speed (measured by the tachometer), the intake manifold temperature and pressure, oxygen content of the exhaust, oil pressure and temperature.  Based on these inputs and the designers' algorithm, the EEC-IV controls the amount of fuel injected, the timing of the injection and the timing of the spark. The development of active computer control of engine operating parameters has permitted many important advances in engine design and performance. In this experiment, we are not going to study how the EEC-IV works, but it is a very important component of engine performance and you will see the impact of the EEC-IV performance.

The Power Needed to Move the Automobile

The amount of power needed to operate the automobile in a cruise condition is small compared to the power that the engine can deliver.  The road load power curve shows the amount of power needed to drive the average car as a function of speed.  Most of the power delivered by the engine is required to overcome the wind resistance.  You can see from the curve that the wind resistance increases dramatically with vehicle speed.  This is why 55 mph speed limits were useful in conserving fuel.

The Efficiency of the Engine

The engine gets its energy from the exploding fuel-air charges in the cylinders. Part of that energy is delivered to the wheels, part of it is lost to the surroundings as heat and part of it is used to operate the engine itself. It is useful to understand what happens to the energy which is not converted to useful work. Decreasing this fraction has important economic and environmental implications.  Energy is lost to friction and pumping work. Friction includes the sliding of the piston and rings, the rod bearings, the valve train, the crankshaft bearings, the oil pump and water pump. The pumping work includes the work done on the air by the engine during the intake, compression and exhaust strokes.  The engine is acting as a pump when it works to suck the air past the partially closed throttle and into the cylinder and when it works to expel the exhaust and compress the air-fuel mixture in the compression stroke.  This work requires energy and reduces the amount of power that the engine can deliver to the dynamometer (or to moving the car).

When the throttle is wide open (WOT), air moves freely to the cylinder on the intake stroke and the largest mass of air possible is available for combustion.  The engine control unit keeps the air fuel ratio nearly constant, so the maximum amount of fuel is injected and the engine is delivers its maximum power.

At low vehicle speeds, smaller amounts of fuel are required to provide the needed power. When the engine is delivering a small amount of power, then its throttle is mostly closed in order to reduce the mass of the air that reaches the cylinders.  The EEC-IV reduces the amount of fuel correspondingly and so less energy is liberated when the mixture is ignited. The engine must work hard to pull air past the mostly closed throttle. The work that the engine does to pull the air past the throttle is part of the pumping work (also called pumping friction).  When pumping work is greater, then a smaller fraction of the work done by expanding gases is delivered to the dynamometer (or wheels in the case of a car). You will calculate the efficiency of the engine for the different throttle settings and compare the results with what you expect from considerations of pumping work.

Since we can measure the fuel flow to the engine, we know the amount of chemical energy that is being supplied to the engine.  We are also measuring the power that the engine is delivering to the dynamometer.  The ratio of the power delivered to the dynamometer to the rate at which chemical energy is being supplied to the engine is the fuel conversion efficiency of the engine.

The Dynamometer, Tachometer and Fuel Flow Measurements

The dynamometer used in this experiment is a water brake type and measures torque supplied by the rotating shaft of the engine.  The dynamometer applies a load to the engine.  And the engine does work on the dynamometer.  The flow of water through the dynamometer serves to cool it.  The dynamometer measures the torque and rotational speed and calculates the power delivered by the engine. 

A tachometer is provided to measure speed of the engine in revolutions per minute, RPM. Engine speed is also measured onboard the engine and supplied to the EEC-IV. Fuel flow can be calculated from the time that the fuel injectors are open.  This time is reported by the DCL to the computer and is reported as FPW1-15M for one bank of cylinders and FPW2-15M for the second bank of cylinders.  These variables give the injector pulse width in milliseconds, ms. We will be calculating the fuel flow from the average of the DCL FPW1-15M and FPW2-15M readings. 

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