Chapter 3: Engine and Vehicle Testing

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Dynamometers. Four essential featuresMeans of controlling torqueMeans of measuring torqueMeans for measuring speedMeans for dissipating power. Fig. 3.1: Prony Brake Dynamometer. Eddy-Current Dynamometers. Eddy-Current Dyno Theory. Eddy-current dynamometers are comprised of a notched disc (rotor) and magnetic poles (stators) around the periphery at a specified gap.The coil which excites the magnetic pole is wound in a circumferential direction.When a current runs through exciting coil, a ma32953

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Chapter 3: Engine and Vehicle Testing

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1. Chapter 3: Engine and Vehicle Testing BAE 517 - Lecture 3

2. Dynamometers Four essential features Means of controlling torque Means of measuring torque Means for measuring speed Means for dissipating power

3. Fig. 3.1: Prony Brake Dynamometer

4. Eddy-Current Dynamometers

5. Eddy-Current Dyno Theory Eddy-current dynamometers are comprised of a notched disc (rotor) and magnetic poles (stators) around the periphery at a specified gap. The coil which excites the magnetic pole is wound in a circumferential direction. When a current runs through exciting coil, a magnetic flux loop is formed around the exciting coil through stators and rotor. The rotation of rotor produces density difference causing eddy-currents to flow to stator. The electromagnetic force is opposite the direction of rotation creating a brake.

6. Fuel Consumption Measurement Volume-Based Flow Measurement Mass-Based Flow Measurement Important Note: CI engines have a return line from the injectors to the tank to handle leakage – must account for the return flow. Return fuel is hot, and may cause problems with supply fuel temperature control. Return fuel mass flow measurement is easy – add a second container to the mass balance for return fuel.

7. Fig. 3.2: Volumetric Flow Measurement Rotameter – variable area flow meters with “float.” Rotameters must be calibrated for fuel viscosity. Corrections for temperature are possible.

8. Mass Flow Measurement Mass balance with beaker and feed pump – a bit cumbersome. Diesel supply of sufficient quantity for test run is suspended on load cells. Load cells signals are sampled and digitized periodically to track fuel ues.

9. Air Consumption Measurement Air-consumption limits ability of engine to produce power – important measurement! Orifice-style flow meters are used to assess air flow rates. Pressure drops across a calibrated orifice is used to specify air-flow rate. Caution -- pressure drop at orifice reduces air flow to engine!

10. Combustion Data Acquisition HDC – head dead center (same as Top Dead Center) Crank rotation should be measured at a resolution of 0.25o or better. Piezoelectric pressure transducers are installed in the cylinder – require water cooling, and high impedance. Pressure reading must be logged at 57.6 kHz (0.25o increments) for an engine operating at 2400 rpm.

11. P-V Diagrams From the recorded crank position readings, cylinder volume is calculated as,

12. Fig 3.3: Typical Data Acquisition System

13. Rate of Energy Release from Fuel Instantaneous energy release from fuel can be approximated using the following relationship,

14. Power Correction for Atmospheric Conditions Power output varies with local atmospheric conditions. SAE Standard J1349 provides an method for correcting engine power to standard conditions. This approach begins with the ideal gas law. Because r, mass density, is mass per unit volume,

15. Power Correction for Atmospheric Conditions Rearranging the previous equation, the ratio of densities becomes,

16. Power Correction Continued Increased ambient air temperature reduces air density. The engine also transfers heat to the air as it enters thereby increasing the density, and therefore the density change is not directly proportional to T-1. The power correction factor for SI engines is,

17. Power Correction Continued Theoretically, the correction factor is applied to indicated power, and therefore,

18. Power Correction Continued Because Pf is often much smaller than Pb, and because fa is often close to 1, the last term of the previous equation is dropped, and the correction is applied as,

19. Power Correction Continued The power correction for CI engines is somewhat more complicated – for example,

20. Power Correction Continued From the previous slide, ff is defined as,

21. Table 3.1: Values for m and n.

22. Power Correction Continued The fm exponent is as follows,

23. Power Correction Continued From the previous slide, F is fuel consumption (kg/h), X is the stroke factor (1 for 2-cycle, 2 for 4-cycle). The value of r is, where pb is the turbocharger boost pressure.

24. Homework Set No. 2 Do problems 3.2, 3.4, 3.6, 3.10, 3.12, 3.14 and 3.15 at the end of Chapter 3 for next Tuesday.

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