In-Cell Services
retain heat within the heat exchanger until a required temperature is attained. The valve is controlled by the test-bed control equipment (a proportional integral derivative [PID]
system, which will be discussed later in this chapter) and is either pneumatically or electrically activated and fails to a safe condition to prevent overheating. The control temperatures can be set on the test-bed equipment, and the Saunders valve responds to the control signals when required.
Note that the control temperatures for the test should be stated clearly in the test instructions.
Fuel, oil, or coolant is passed into the casing and imparts its heat to the raw cooling water flow in the tubes, which run from end to end. The raw cooling water runs through tubes absorbing heat from the fuel, oil, or coolant flowing around them.
3 0 An Introduction t o Engine Testing and Development
Air Temperature
The temperature of the air entering the inlet manifold has a direct influence on the complete integration of the air and fuel as a mixture, with regard to the evenness and atomization of the mixture (i.e., the dispersal and size of the fuel droplets throughout the air stream and on the mass of the airlhel mixture). When the air temperature is too low, fuel does not mix as effectively with the air stream due to the higher density of the air; therefore, it tends to fall to the sides and floor of the inlet tract. This gives an uneven mixture and poor atomization, which may cause misfiring and so forth. In turn, this can lead to higher emissions of hydrocarbons and carbon monoxide. In the case of air temperatures that are too high, the air charge is expanded by its heat and, as a consequence, has a reduced density. This reduced density means that the fuellair charge will have a lesser mass than preferred (lower total oxygen content).
Note that the power developed by the cylinder of an engine is proportional to the mass of the fuellair mixture drawn into the cylinder. If the temperature of the fuellair charge is too high, then the maximum power that the engine will achieve is reduced.
The optimum combustion or induction air temperature obviously lies someplace between the two extremes and may vary with the engine or management system fitted for the test purpose.
Vehicle induction tracts may have a temperature control device fitted into the air filter intake pipe and connected to the exhaust manifold heat shield, to transfer heat to the air filter temperature control device and to help maintain a consistent desired temperature range in the air entering the engine. The device usually is a flap operated by a tem- perature sensing mechanism that progressively lets hot circulated air (drawn from the area of the exhaust manifold) into the engine inlet tract to warm the induction air. The test engineer should ensure that, where applicable, the required air temperature figure is stated clearly in the test instructions before starting the testing; otherwise, the results might be questionable.
The engineer must ensure that the air inlet temperature remains constant throughout the test as required and should pay due regard to positioning of equipment that may affect the temperature control (e.g., spot coolers).
Measuring of the air temperature can be done by situating the sensing thermocouple either in the inlet tract or in the free space near the opening to the tract, where it will record the ambient air temperature in the area where air is drawn into the engine inlet tract.
Air Pressure
Combustion or induction air pressure has a direct bearing on the performance of the internal combustion engine. The mass of the fuellair mixture charge within the inlet manifold ready to be drawn into the cylinder can be increased or decreased by a high or low air pressure, respectively. The power developed by the cylinder of an engine is proportional to the mass of the fuellair mixture that is drawn into the cylinder. An example in appreciating the idea of air pressure is in the supercharging and turbocharging of engines. Both pressurize the air (among other things), forcing more airlfuel mixture (or air alone in the case of diesel engines) into the cylinders in the short time allowed by the inlet valve opening period, thereby increasing the mass of the charge and increasing the maximum attainable power.
If the test engine is inducting its combustion air from the ambient air in and around the test cell area, then it will be vulnerable to changes in the atmospheric conditions that
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In-Cell Services 3 1 affect air pressure. In many test cells, barometers are a common sight, and the former
practice was that frequent readings of the ambient pressure would need to be recorded and then be taken into account when recording the test data. The current practice is to continuously monitor the air pressure within the cell, with automatic correction calcula- tions done by the data acquisition equipment. Improperly sited spot coolers can have the effect of ramming the air into the air inlet tract if positioned too close to the opening to the tract. The ramming effect increases the turbulence of the air and thus its pressure.
This may lead to inconsistent test results over a period of time, especially if the cooler is moved later during the test. Air turbulence at the induction area also can steal air from the induction process. Turbochargers and superchargers are introduced to increase the mass of the charge of airlfuel (or air only for diesels) entering the inlet manifold.
Note that in typical road vehicles, the air generally enters the inlet tract to the air filter under ram air conditions as the vehicles move along the road at speed, pushing through the surrounding air. Depending on the air speed, this effect normally is small (e.g., 0.1 bar at 60 mph).
Air Humidity
In general test cell operations, the humidity of the air admitted to the cell and entering the engine air inlet is not controlled (i.e., it is that prevailing in the general atmosphere).
As legislated emission standards become more stringent, the control of humidity within the cell becomes a key issue because the degree of humidity affects the NOx emissions and the formation of particulate platelets in the combustion space. Both humidity and air induction temperature must be controlled in order to replicate tests at some time in the future. In the case of non-climate-controlled basic test cells, where air temperatures are concerned, there is quite a lot that the engineer can do to control the conditions within the test cell area. One example is ventilation.
Ventilation
The efficient flow of air through and around the test cell is important for the safety and reliability of the testing operation. Although airflow rate and extraction through the cell usually are controlled automatically, the flow around the assemblies and equipment within the test cell is affected by the care and thought applied by the technician when setting up the test. It is important that cells are kept tidy and clear of unnecessary equipment, particularly near the ventilating fans and so forth.
Electrical Equipment
With so much sensitive equipment located in and around the test cell, individual appli- ance positioning is vital, particularly where numerous items of electrical equipment are located together, each generating its own heat outputs. Most electrical appliances usually have a ventilating panel to aid cooling of the components inside. Some thought should be given as to how best to position these appliances to help their cooling, thereby reducing the potential for hazards and faulty equipment.
The fact is that electrical appliances cause most in-cell fires due to faulty cabling, incor- rect installation, poor ventilation, or simple misuse and abuse!
With regard to basic test cells, the problem for the engineer is to understand and remove, as much as possible, the potential causes of increases in humidity. The technician also should consider humidity. The local buildup of humidity to excessive levels would have Previous Page
32 An Introduction t o Engine Testing and Development
detrimental short- and long-term effects on electrical-equipment-measuring instrumen- tation being used during the tests. Failure to appreciate this fact can lead to cases of expensive equipment failures, as well as time wasted in analyzing test results that owe more to faulty instrumentation than to the characteristics of the engine. When testing prototype engines and components, total confidence in the measurement instrumenta- tion is of the highest importance. Awareness of the effects of excessive temperatures in the cell, and the further complications this may cause, will help to minimize potential problems.
Air Condition (Quality)
Thought also should be given to the effect on air quality of fuel and exhaust leakages within the test cell area, which are not only detrimental to the test results but also are potentially dangerous and even lethal to the technician.
As would be expected in an industry that tests engines to extremely high specifications and uses expensive and sensitive equipment (not to mention expensive engines), the need for vigilance in all aspects of testing is of the utmost importance (e.g., good fuel, good conditions, good practices, good technicians).
Air Cooling and Ventilation
Air as a cooling medium is not as effective as a liquid cooling system, particularly where the cooling of hot surfaces is concerned, as those surfaces are in engines and dynamometers running flat-out under load. The principal reasons for this are that air generally has a lower density than liquid and is considered to be invisible to radiant heat; in effect, this means that the air has poor heat transfer qualities under normal cir- cumstances. However, with high-velocity air, as in spot cooler fans, the heat transfer rises considerably, as one would expect. But beware of the problems that spot cooler fans can cause, as discussed in the heat exchanger section, particularly with regard to position and air turbulence near the inlet tract intake.
Consider in the following list the heat output that each will contribute within a test cell, where the engine may run at full speedlfull load for lengthy periods:
Engine Exhaust
Dynamometer-AC or DC motors Shaft and couplings
Electric lighting Spot cooler motor
Ventilator fans and motors
All of these contribute to making the test cell a very hot place in which to operate at times. Good, effective ventilation and cooling are vital for safety reasons as well as for test result reliability. Likewise, engineers and operators require a convivial work place, as shown in Figure 2.9.
Figures 2.10 and 2.11 illustrate the various types of testing from anechoic to large engine test work.
The main components of the process enclosure (Figure 2.12) of the fuel temperature conditioning unit are as follows:
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Figure 2.9 Control area of an engine test facility.
(Courtesy of Froude Hogman)
Figure 2.10 Anechoic test cell, (Courtesy of Froude Hofian)
Circulating pump Selector valves Header tank Immersion heater
Flat plate heat exchanger fed by chilled water circuit Temperature and level sensors
The control enclosure marshals the mains switching and inputloutput (110) data transfer associated with the control of the process enclosure. The process enclosure requires one set of feed and return lines for the process water circuit and one for the chilled water circuit. This minimizes the pipe-work in the test cell. The temperature in the header tank is maintained or adjusted by directing the intake closed loop water.