A desirable feature of vehicles having an automatic transmission containing a fluid torque converter between the engine and the driving wheels is "creep" this being a motion, normally experienced on flat ground, either forward or backward, of the vehicle when the brake thereof is released, the vehicle is "in gear", and the engine speed is at idle. Such vehicle creep is considered a desirable feature in a vehicle as it allows the driver to achieve a slow, steady movement of the vehicle by merely releasing the brake and without having to depress the accelerator or throttle of the vehicle. Thus, by operation of the brake alone, the driver can selectively achieve a slow forward or rearward motion of the vehicle or maintain the vehicle stationary when it is in gear. This nature of operation of a vehicle is convenient and desirable in various traffic conditions and particularly in congested, slow moving traffic wherein the ability to achieve small movements of the vehicle without the need for the driver to move their foot away from the brake or to actuate the accelerator pedal is very desirable. It also provides the vehicle with a positive feel during maneuvering in traffic and when parking and prevents vehicle roll back to a certain degree during hill starts.
Vehicle creep is generated by the torque transmitted through the torque converter when the engine is idling in, for example, "drive". The main criteria used to select a torque converter for an engine having an automatic transmission is the engine torque characteristic. If the engine has relatively high low-speed torque, the torque converter will have a low stall speed therefore providing a "stiff" coupling. However, if the engine torque is generated at a higher speed, the torque converter will have a higher stall speed providing a "loose" coupling. This ensures that the engine speed is kept close to the speed that gives the highest torque during the commencement of movement of the vehicle and during general driving so as to achieve the best vehicle performance. Therefore, the level of creep is governed by the torque converter "stiffness" and the engine idle speed.
A number of criteria affect the selected engine idle speed for a vehicle and engine combination. These include body vibration due to any resonances caused by engine vibrations, combustion stability at low engine speeds, the accommodation of loads such as power steering, air conditioning and neutral drive transitions, and the amount of vehicle creep desired.
In a vehicle having an automatic transmission, when the vehicle is stationary and in "drive", the engine is loaded by the torque converter as it tries to rotate the input side of the torque converter with the output side of the torque converter being held stationary. Therefore, the higher the engine idle speed, the higher the loading on the engine, and the more energy, and therefore fuel, that is wasted primarily in the form of heat into the gearbox oil. Therefore, idle speed on an automatic vehicle has a significant effect on fuel economy. However, where a low idle speed is used in order to limit fuel consumption and exhaust emissions, that idle speed may be too low to achieve creep, or a desired level of creep.
In addition to not providing the desired level of creep, such a low idle speed may lead to undesirable shudders or jolts in the vehicle drive train if the driver depresses the accelerator harshly to rapidly increase engine speed from this low idle speed. This is due to the fact that at such low idle speeds, there is very little or no load appearing in the vehicle drive train including the torque converter. Accordingly, under hard acceleration, the unloaded engine is able to rapidly gain momentum until the speed thereof approaches the stall speed of the torque converter, at which point the engine suddenly takes up the torque converter load and subsequently transfers a shock through the drive train. This may be further accentuated as the drive train may become unloaded to the point that there no longer exists a pre-load between the various drive train components. In such a case, clearances may need to be taken up during the initial increase in idle speed which may lead to a noticeable physical shudder and audible impact.
In the case of vehicles having manual transmissions, it is also desirable to set the engine idle speed as low as possible. This however presents the problem that engagement of the clutch by the driver may stall the engine if the engine idling speed is too low.