Usage:
3406 Truck Engines
The drivability of a truck engine depends on both the power that an engine delivers to the wheels, and how responsive the engine is to throttle movement. The most powerful engine will not be drivable if it is not responsive during normal driving conditions. As of this time, a satisfactory response test has not been developed for a chassis dynamometer.
Caterpillar's definition of engine response is the length of time required to go from a no load (idle) condition, at some engine speed, to eighty five percent of the available power at that given speed in a laboratory environment. On engine dynamometers, this is done by measuring the actual torque that the engine delivers during a full throttle constant speed acceleration. In a vehicle, it is not possible to measure power or torque accept on a chassis dynamometer and the chassis dynamometer is not responsive enough to hold a constant speed during an acceleration. The response test is extremely sensitive to variations in engine speed, so it is imperative that the engine speed be kept as constant as possible during the testing.
A test method has been developed to test dynamic response in a vehicle so that different vehicles can be compared or so that repairs on a vehicle can be evaluated. This test method can also be used in conjunction with the PAR dynamometer test to come up with a more complete picture of the overall performance of an engine in a chassis.
Since this response test is not run on a chassis dynamometer, a method of measuring engine output needs to be established. A good relative power measurement on a turbocharged engine is turbocharger boost. Turbocharger boost can be measured with a boost test kit or any other gage that is responsive and has a full range of 175 kPa (25 psi). Gages with larger ranges will not have the sensitivity required to run the test.
Due to the sensitivity of the response test to engine speed, it is necessary to have some weight attached to the vehicle. A tractor without a weighted trailer attached will usually not provide an acceptable answer because the tractor will accelerate several miles per hour during the test. This will not give an acceptably consistent results.
This response test should be run on a level road where the vehicle can be operated safely at 1100 rpm in the highest gear. Once you are familiar with this test, it can be completed in about ten miles of driving.
After the truck has been equipped with the boost gage and a test trailer, it should be driven for a length of time prior to starting the test. This will ensure that the engine is up to operating temperature. After the engine is up to operating temperature, the data collection can start. For the best results, data should be taken at four engine speeds (1100, 1200, 1300, and 1400 rpm). This will allow you to develop a response trend when the results are plotted on graph paper. (See the illustration.) Any change to the engine can then be compared back to the first base line run.
When running the response test on an electronically controlled engine, the engine should be run for several minutes to allow the engine strategies to stabilize before collecting the response data. This simulates what an operator would see during over-the-road operating conditions.
If the vehicle has a compression brake, the compression brake should be disabled because it could affect the results of this test. After the vehicle is brought up to operating temperature, the transmission should be placed in top gear and the engine run at 1100 rpm. The throttle should then be advanced to the full power condition. After the engine reaches 1150 rpm, the throttle should be reduced to idle and the vehicle allowed to coast down to 1100 rpm. The coast down should not be allowed to take more than 10 seconds. When the engine reaches 1100 rpm, simultaneously apply full throttle and start a stopwatch. When the boost pressure reaches 51 kPa (7.5 psi), stop the stopwatch and record the time. (A small tape recorder works well for recording the time because it may be difficult to write legibly in a moving truck.) The process should be repeated several times until at least three consistent readings are recorded. Then the process should be repeated at the next higher rpm until you have readings from 1100 through 1400 rpm. After the data has been collected and you have returned to the shop, the data should be written down and averaged for each rpm. The averaged data should be plotted with the engine rpm on the X-Axis and response time on the Y-Axis.
Because of the variability of different gages and slightly different techniques in running the test, it is difficult to give absolute values for the response time at a given engine rpm. But, a good rule of thumb is that the response time at 1200 rpm should be around 3 seconds. This test is very good at sorting out vehicles where customers complain about a "good truck" and a "bad truck". With the PAR dynamometer test and this test, you can cover the basic drivability of a vehicle very well.
See the illustration. The graph shows a plot of engine rpm versus response time. A second set of data could be plotted on top of this original data to show improvements to the engine drivability as a result of work done on the vehicle. This plot along with a PAR dynamometer run would give a complete picture of the overall acceptability of performance for a given engine.
