Excelsior Uses NI LabVIEW and DAQ for Pressure and Combustion Analysis

Author(s):
Barrie Hall - Excelsior Engineering Ltd

Industry:
Automotive

Products:
LabVIEW,

The Challenge:
Investigating the normal combustion process, including the knock phenomena, and carrying out the vehicle and engine analysis on a test bed while reducing the total cost.

The Solution:
Using National Instruments LabVIEW software and data acquisition (DAQ) to capture the normal cylinder combustion pressure and knocking events over an appropriate time period while also manufacturing electronic hardware to take and prepare the cylinder combustion pressure signal.

The Importance of Proper Design and Manufacture

In a modern high-compression gasoline engine, engineers must show care in design and manufacture to ensure the combustion process proceeds in a quick and controlled fashion. The fuel economy, performance, and emissions must remain stable over the engines speed and load range, as well as production manufacture tolerance variation. Fuel quality, also known as octane rating, within the petroleum industry worldwide can change within the legal requirements due to the mix of crude oil as well as the requirement of better starting in colder weather and in summer, lower volatility fuel. Engineers must include additives in the fuel to improve heat release, octane rating, antifoaming of the fuel, and reduce intake and exhaust valve clogging.

Cheaper, lower-octane gasoline fuels are available at gas stations and generally have a lower-octane rating. These lower-octane fuels also cause the combustion-end gas to self ignite ahead of the combustion flame. This end-gas self ignition causes a pressure wave to be set up and can be heard externally as a ringing noise, the sound known as knocking or pinking. The knock pressure causes the local erosion of the metal, which overtime causes engine failure. A second form of engine damage is caused by exceeding the design limit of the piston, piston pin, and con rod. Finally, a third type, known as preignition of the fuel/air mixture in the engine due to a local hot spot, causes immediate failure of the piston.

The engine calibrator can minimize the effects of knock from fuel variation and ensure the design maximum cylinder pressure is not exceeded by adjusting the point of ignition. This may require the user to obtain data either from an engine in a controlled test cell where environment and fuel types can be varied or in a vehicle application in different atmospheric and altitude testing. Hence, there is a need for suitable analysis equipment to confirm calibration robustness and the task undertaken.

We took the combustion pressure signal, filtered pressure signal, accelerometer signal, and ignition advance signals through the DAQ card into the laptop. This data is kept as part of a sign-off document to confirm calibration readiness and design limitations. We used the LabVIEW software for statistical distribution graphs for the critical inspection of the data and to confirm the acceptability.

We displayed the data for cylinder pressure, knock pressure, knock energy, and ignition advance as four separate graphs. The scatter of cylinder pressure was always large and caused by cyclic variations in the combustion caused by air/fuel mixing or preparation and turbulence in the combustion chamber. Although the variations are large, the scatter of data is not sufficient for the data to go over the critical 5 bar and 75 bar pressure design limits, and therefore, at this engine, running condition passed as acceptable.

Dynamometer Alarm and Shut Down

We continuously took the cylinder pressure and filtered cylinder pressure data for all cylinders using LabVIEW, emptying and filling displays to indicate levels of knock and cylinder pressure. The system gave a simple audible warning as the operator may not be viewing the data at the time it is displayed. Finally, on reaching an unacceptable level, a digital pulse was sent from an output DAQ channel to the engine test cell computer to request an engine shutdown. The data held within the LabVIEW display indicates to the operator whether it shut down due to high cylinder or knock pressure and which cylinder shut down.

The aim of the project was to provide data collection, analysis, and storage of either the raw data for future comparison or engine calibration sign off fit for production or printouts. By installing LabVIEW on a laptop, we can take data in the vehicle on the test track or in the test chamber. Using NI products, we saved approximately $100,000 to $150,000. We also saved development time by finding design issues early in the engine testing, having the 3-D graph to compare data and then confirming subsequent redesigns.

Our ability to have one operator working several engine dynamometer test cells saves money especially at night and weekends. With the alarm program, they system performs a safe shutdown by sending a stop-engine signal to the test cell console as well as an audible warning as failure begins. The alarm saves postdata, which gives an indication as to which cylinder of the engine is experiencing concern. This saves money and time later, as the engine still contains various mechanical parts for inspection in an undamaged form making diagnosis easier. The engine can be recovered and rebuilt rather than having to be scrapped, which can be a large expense of prototype engines.

For more information, contact:

Barrie Hall

Excelsior Engineering Ltd

53 The Chase

Rayleigh Essex, England SS6 8QW                                                                                    

E-mail: bhall@knock.fsnet.co.uk

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