Table 3.3shows a possible energy balance sheet for a cell in which a gasoline engine is developing a steady power output of 100 kW.
Control surface Control volume In
Fuel Air Water
Out
Machine
Exhaust Power Water
FIGURE 3.1 An open thermo- dynamic system.
2. Compressed air may be considered as an energy input in facility design; however, for individual cell energy flow purposes its use tends to be intermittent and makes an insignificant contribution.
Note that where fluids (air, water, exhaust) are concerned the energy content is referred to an arbitrary zero, the choice of which is unimportant: we are only interested in the difference between the various energy flows into and out of the cell.
Given sufficient detailed information on a fixed engine/cell system, it is possible to carry out a very detailed energy balance calculation (see Chapter 6 for a more detailed treatment). Alternatively, there are some commonly used
“rule-of thumb” calculations available to the cell designer; the most common of these relates to the energy balance of the engine, which is known as the
“30–30–30–10 rule”. This refers to the energy balance given inTable 3.4.
The key lesson to be learnt by the nonspecialist reader is that: any engine test cell has to be designed to deal with energy flows that are at least three times greater than the “headline” engine rating. To many, this will sound obvious but a common fixation on engine power and a casual familiarity with, but lack of appreciation of, the energy density of petroleum fuels still lead people to significantly underrate cell cooling systems.
TABLE 3.1 Inflows and Outflows To and From a Test Cell
In Out
Fuel Ventilation air
Ventilation air (some may be used by the engine as combustion air)
Exhaust (includes air used by engine)
Combustion air (treated) Engine cooling water
Charge air (when separately supplied) Dynamometer cooling water or air
Cooling water Electricity from dynamometer
Electricity for services Losses through walls and ceiling
TABLE 3.2 Inflows and Outflows To and From an Engine
In Out
Fuel Power
Air used by the engine Exhaust
Cooling water Cooling water
Cooling air Cooling air
Convection and radiation Chapter | 3 The Test Cell as a Thermodynamic System 43
Like any rule of thumb this is crude, but it does provide a starting point for the calculation of a full energy balance and a datum from which we can evaluate significant differences in balance caused by the engine itself and its mounting within the cell.
TABLE 3.3 Simplified Energy Flows for a Test Cell Fitted with a Hydraulic Dynamometer and 100 kW Gasoline Engine
Energy Balance
In Out
Fuel 300 kW Exhaust gas 60 kW
Ventilating fan power 5 kW Engine cooling water 90 kW Dynamometer cooling water 95 kW
Ventilation air 70 kW
Electricity for cell services 25 kW Heat loss, walls and ceiling 15 kW
330 kW 330 kW
The energy balance for the engine is as follows:
In Out
Fuel 300 kW Power 100 kW
Exhaust gas 90 kW
Engine cooling water 90 kW Convection and radiation 20 kW
300 kW 300 kW
TABLE 3.4 Example of the 30e30e30e10 Rule
In Via Out Via
Fuel 300 kW Dynamometer 30% (90þkW)
Exhaust system 30% (90 kW) Engine fluids 30% (90 kW)
Convection and radiation 10% (30 kW)
First, there are differences inherent in the engine design. Diesels will tend to transfer less energy into the cell than petrol engines of equal physical size. For example, testers of rebuilt bus engines, which have both vertical and horizontal configurations, often notice that different models of diesels with the same nominal power output will show quite different distribution of heat into the test cell air and cooling water.
Second, there are differences in engine rigging in the cell that will vary the temperature and surface area of engine ancillaries such as exhaust pipes.
Finally, there is the amount and type of equipment within the test cell, all of which makes a contribution to the convection and radiation heat load to be handled by the ventilation system.
Specialist designers have developed their own versions of a test cell soft- ware model, based both on empirical data and theoretical calculation, all of which is used within this book. The version developed by colleagues of the author produces the type of energy balance shown inFigure 3.2.Table 3.5lists just a selection, from an actual project, of the known data and calculated energy flows that such programs have to contain in order to produceFigure 3.2.
Such tools are extremely useful but cannot be used uncritically as the final basis of design when a range of engines need to be tested or the design has to cover two or more cells in a facility where fluid services are shared; in those cases the energy diversity factor has to be considered.