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2-stroke small engine is the most popular engine as compared to the 4-stroke engine. Eventhough the 2-stroke is small in size that 4-stroke, this egnine can give almost double of the power that can be generated. This is due to the number of complete cycle to generate from pumping the fuel to the exhaust chamber.
Basic concept of engine performance
Engine performance characteristics are conventionally presented of an engine performance characteristics. During the actual data from the actual test run of the engine is obtained and is very useful if comparing to the method by comparing one engine from another engine. In this section, some of important characteristics of SI engine are discussed with more specific.
We can noted that there will be a maximum power output per cylinder per cycle that occurs during certain speed of the particular engine. During this point of cycle, maximum value of force can be exerted to the piston. For practical purpose that during this point, we can say that the torque produced or engine capacity to do work will simultaneously maximum at this position of piston.
Foundation of engine performance
Engine performance is a relative term that is normally represented by typical characteristics curve which functions of engine operating parameters. The term performance usually means the relationship between the power output, revolutions per minute, fuel or fluid consumption and ambient conditions in which an engine operates or how effectively it is provides useful energy relation to some other camparable engines.
Most of the engine testing for their performance characteristics takes place under laboratory condition. The engine is connected to a power ââ‚¬"absorbing device called dynamometer (G. P. Blair, 1996). However, the performance characteristics of power, torque, fuel consumption rate, and air consumption rate, at various engine speeds are recorded (J.B. Heywood, 1988). There are so many typer of dynamometers; the principle of any dynamometer operation os to allow the casing to swing freely ( A.J. Martye and M.A. Plint, 2007). The reaction torque on the casing, which is exactly equal to the engine torque, is measured on a level length, L, from the centerline of the dynamometer as force, F. This restrains the outside casing from revolving, or torque and power qould not be absorbed (G.P. Blair, 1996 and A.J. Martyr and M.A. Plint, 2007).
Figure 2.1: example of diesel engine test rig
According to reference, engine performance theory could be divided to tho areas such as to learn performance engine using dynamometer (G. P. Blair, 1996, J.B. Heywood, 1988 and A.J. Martyr and M.A. Plint, 2007), and to repair and maintain engine through diagnostic analysis system with the result is the efficient operation of engines of all the design (J. Erjavec, 2006). Both theories have different objectives. First, to learn performance engine used dynamometer. The goal is could be developed engine performance or may ber also give service to customers for selected en engine. Second, is to monitoring of engine in order to have the efficient operation using diagnostic technique.
Then there are several factors that must be considered in evaluting the performance of the engine. Most of them are maximum power or torque available at each speed within the useful range of speed. Some range of the power output at constant speed for stable operation of the engine must be decided. The different speeds should be selected at equal intervals within the useful speed range. After that, specific consumption at each operating condition must be specified within the useful range of operation.
Dynamometers have been widely used for evaluation the performance of many kinds od internal conbustion engines. Many different types of dynamometers systems are in use today and detailed descriptions of the various types can be found in many book. While dynamometer make measurements of engine performance at specific loads, a large number of numerical simulations have also been developed to model the engine cycle and predict engine performance. An important advantage of these codes is that they allow the researcher to investigate the effects of specific aspects of the operating physics like friction, heat transfer and the chemistry involved
in the combustion process.
The importance of engine performance testing
The two stroke engine was developed only in two opposite fields that are marine engines which are used a separate scavenging pump and small engines for various application such as power tool. The two stroke cycle engine was invented by Sir Dugald Clerk in England at the end of the 19th century. The form of the engine is using the crankcase compression for the induction process, including the control of the timing and area of the exhaust, transfer and intake ports by piston (G. P. Blair, 1996)
Todays, the most common power source is the well known two-stroke engine. This type of engine has been natural choice due to low cost ang high power density. The two-stroke engine, as we know it today is not able to comply with future emission demands due to its massive hydrocarbon pollutant. However, in the future engine manufacturer must come up with new cost efficient engine technologies that still deliver the same or improved performance for customer satisfaction (M. Bergman, 2003).
Power and torque measurements obtained at different loading conditions show the general trends for most convectional scale engines. Based on figure 1, both parameters increase with increasing engine speed and reach a flat peak before decreasing at higher speeds. Engine torque is usually found to peak first followed by a peak in the engine power. Transmission systems in automobiles and other applications are designed to allow the engine to operate in the relatively narrow where it yields maximum power, torque and efficiency.
Figure 1: typical shape of power, torque characteristics of a conventional 2 or 4 stroke engine
Waltermann and Neundorf from University Padervorn, Germany, have been developed a test bed for the design and optimization of a series hybrid drive train (P.Waltermann and N Neuendorf, 2006). For the test bed control a powerful real-time environment is applied that comprises component control and the simulation of different models as well as the operating strategy and the emergency system. All important component of a series hybrid drive are mounted. The individual components (combustion engine, generator, battery and traction drives) were contolled with different modules.
Dynamometer is a device to measure force, torque, or power. For example, power produced by an engine, motor or other rotating prime mover can be calculated by simultaneously measuring torque and rotational speed (rpm). In standard emission testing cycles, dynamometers are used to provide simulated road loading of either the engine (using an engine dynamometer) or full powertrain (using a chasis dynamometer). In fact, beyond simple power and torque measurements, dynamometers can be used as part of a test rig for variety of engine development activities such as the calibration of engine management contollers, detailed investigations into combustion behavior.
Air flow measurement
For the air flow rate consumption, there are a few methods that can be used to make an approximate estimate of the air mass flow rate supplied to the engine.quasi steady flow rate is meant by the average air flow rate over the entire engine cycle because usually researchers are not so interested in the instantaneous air flow rate into the engine. We can divide the total air supplied to the engine by the time taken that we recorded. Among the method used to measure the air flow rate measurement are air box with sharp edged orifice plate, viscous flow meter, positive displacement flow meter and corona discharge flow meter.
Figure 2.2: flow passed the orifice plate
For this experiment, we are using air box with sharp edged orifice plate because it is the simplest quasi-steady air flow rate method is the air box. The air drawn from the box by the engine is replenished by atmospheric air which enters the box through a calibrated sharp-edged orifice plate or a venturi. The volume of the air box must be sufficiently large to damp out the air pulsation generated by the engine, so that the air flow through the orifice plate is as far as possible steady. The air mass flow rate through orifice plate can be calculated by using formula given above:
áÂ¹Â=CdA âË†Å¡(2âË†â€ p) kg/s
This equation is applicable to incompressible flow. Normally Cd=0.6 is used will sufficiently accurate for most purpose. In this experiment for this spark ignition engine, the air flow rate is minimun when the engine is idling ( small opening of the throttle) and maximim at high load and at the maximum opening of the throttle.
Flow meter is a device that used to measure the stream of waterr in the system. There are two type of flowmeter that is horizontal flowmeter and vertical flowmeter. There are a variety of size of flowmeter that need to be choosen appropriately before being used to make sure that we are using the correct size of flowmeter. Before being decided to buy the flowmeter, one must know the maximim value of the flowrate that usually being recorded in gallon per minute, GPM. The brochure usually comes in terms of pipe size, flow range and display model option. By hook or by crook, we must be able to provide the flow range of the stream because it is very important to know.
Fuel flow rate measurement
There are various method to measuring ht efuel consumption rate for an angine. Burette method is the simplest way to measure the fuel rate consumption because it will depending on the time that we run the engine for it to consume a certain volume of fuel. This type of masurement is more economical and will give more accurate result based time that engine is running.
Another way to measure the instantaneous fuel consumption is by using the weighing methodfor measuring the instantaneous fuel mass rate that had been supplied to the engine. This is a responsive and can be monitored without need to be turn off the engine. It used the hydraulic equivalent of the electrical wheatstone bridge. The bridge is made up by four orifice and the pump to establishes a reference fuel flow which recirculate around the bridge.
When there is no fuel enter the bridge, the reference fuel flows through the bridge is balanced. However, when fuel passes through the bridge on its way to the engine, the bridge become unbalance and it produces a pressure difference signal which is directly proportional to the instantaneous mass flow rate of the fuel passing through the bridge. When the accuracy is not the main point in the measurement, we can used rotameter as the mechanism to measure the fuel flow rate.