Onboard system tests diesel aircraft engine

John Gyorki, IOtech -- Test & Measurement World, 9/1/2005

Before a new engine can be approved and commissioned for general and experimental aircraft, it must log hundreds of operating hours on a dynamometer test stand without failure. Unfortunately, the work needed to successfully carry out such an undertaking can be daunting for an engineering crew. A crew member must be present during the entire test, even when the event runs around the clock.

The test situation became quite acute for the various teams of engineers, technicians, and volunteers at DeltaHawk Engines in Racine, WI, when the company began endurance testing a new 160-hp, four-cylinder diesel engine. Test observers were required to make minor adjustments or fine tune the test setup on the fly any time a measured variable from one of the numerous sensors placed around the engine exceeded a preset limit. In addition, the equipment had to be relatively easy to program and operate by people with a wide range of skill levels.

The company had used conventional temperature, pressure, and flow gauges in the past, but this approach did not provide the trend data for future analysis. Also, during initial tests, the engineers employed a data-gathering system that worked well, but the desktop computer and rack-mounted hardware were too large and heavy to use on the aircraft for in-flight tests. Moreover, the equipment was relatively difficult to program and maintain during a test—it runs most reliably under the hands of a programmer and other skilled technicians and thus was not practical for the less-skilled staff members who manned the tests at odd hours of the day and night.

The company found a more compact solution in an IOtech Personal Daq data-acquisition system connected to a laptop computer. This portable system runs with 12-VDC input and gathers the data from an engine and dynamometer on a nearby trailer. Because the Personal DaqView software displays virtual gauges on the laptop screen with programmed, color-coded upper and lower test limits, operators of all skill levels can easily see when a variable crosses over the color-coded limit lines.

One critical variable that the DeltaHawk team must measure is the oil pressure, which normally runs between 20 and 100 psi over a specific range of engine speeds during extensive testing. The team must monitor the oil pressure across numerous temperatures while checking for component wear and cracks. Eventually, the team dismantles the entire engine and then puts it back together. The DeltaHawk team performs the tests on two engines, one of which usually runs in an airplane while another runs on the trailer. In the plane, the test equipment charts 10 channels of sensor data with a laptop computer. On the trailer, data are collected through 20 channels.

Typically, USB lines run from the engine area to the laptop computer on a table over 25 ft away. The only electronic components on the engine are the sensors collecting the data. This engine is a straightforward diesel and does not require sophisticated electronic ignition systems or computers. Even the fuel pump is mechanical, but an alternator charges the battery that runs the boost pump. If the system should lose power in flight, the pilot's instruments would be the only components that fail. The plane would still fly just as reliably without them.

The engine is a four-cylinder V-configuration. The test system lets the test team monitor the engine speed and the temperature of each cylinder head in a water jacket, which is divided into two sections between the one/

three cylinder bank on one side and the two/four cylinder bank on the other side. A water temperature sensor in each bank monitors the respective temperature. Since the engine has both a turbocharger and a super charger, the exhaust gases are monitored with a sensor in the inner blower, between the two chargers. Pressure sensors record inner blower pressure and manifold pressure under the supercharger. The system measures oil temperature as the oil goes into the engine and again as it exits the engine on its way to the heat exchanger.

For the engine installed on the plane, the engineers also monitor static and dynamic pressures. The static pressure is the ambient pressure, and the dynamic pressure is the air pressure inside the air scoop on the bottom of the plane—the air over the heat exchanger. Test programs vary sensor number and placement. The team analyzes the data with a spreadsheet that indicates the length of the run and rpm level. The system archives charts of data that the team can use to compare all the runs made with one engine or several engines.