Depending on your transmission model, the gear split may vary from 400 to 500 rpm. The electronic governor provides almost no overrun capability; and, if the transmission is downshifted too early, you will experience a temporary loss of pulling power until the engine speed falls below rated speed.
In general, when using a 7- or 9-speed transmission, you should always downshift between 1000 and 1100 rpm. This is true even on steep grades with heavy loads. When using an 18-, 15-, or 13-speed transmission, you will need to downshift at an rpm that allows “less than rated” rpm before throttle application in the next gear down. You may want to limit engine speed to 1900 rpm in all gears. The DD15 engine provides horsepower through 2100 rpm, but fuel economy is not as efficient above 1800 rpm.
If you decide to drive at lower rpm’s for improved fuel economy, don’t let different engine noises throw you off guard. The DD15 engine sounds quiet at 1400 rpm, almost as if it had quit pulling. If you had a boost gauge to look at while driving, you would notice the turbocharger maintaining steady intake manifold pressure, even as rpm falls. Depending on the air intake arrangement, you may also experience a “chuffing” sound as the engine starts to pull hard at lower rpm’s. This is normal and caused by the velocity changes of the air flow within the air intake plumbing. Electronic engines can actually deliver more fuel at lower engine speeds than at rated speed.
The DD15 engine has been designed for a very quiet operation, but the air flow may be noticeable to the tuned attentive ear. The turbocharger operates at higher boost pressure forcing EGR gas flow through the EGR plumbing. In some situations the driver may believe he/she has experienced a charge air cooler system leak. Even connecting trailer light and air hoses, the driver may hear a different tone (exhaust and under hood with the engine idling.) If equipped with a turbo boost gauge, the driver may occasionally note intake manifold pressure exceeds 35 psi (6.89 kPa).
|EPA07 DD15 Operator’s Guide – DDC-SVC-MAN-0003|
|Generated on 10-13-2008|