1. Field of the Invention
This invention relates to an apparatus and method for testing both two and four wheel drive vehicles (2 WD and 4 WD, respectively) having a variety of drive train configurations under anticipated road conditions.
2. Description of the Prior Art
Test apparatus in the form of dynamometers is widely used for testing motor vehicles in place. Since the test vehicles are not moving over a road bed, the dynamometer must simulate certain forces normally associated with actual vehicle operation. These parameters include forces associated with inertia (related to the mass or weight of the vehicle) and road load forces (related to the velocity of the vehicle). The vehicle engine (or its braking system) must overcome inertial forces in order to accelerate or decelerate the vehicle In addition, the engine must overcome breakaway frictional and rolling frictional forces (i.e., road/tire friction) as well as windage forces (i.e., drag forces caused by air passing over the vehicle). These latter forces are commonly referred to as road load (RL) forces and may be represented by the formula: EQU RL=A+BV+CV.sup.2 +DV.sup.3 +EW
where A represents the effects of breakaway force, and, B, C and D represent the effects of rolling friction and windage, V represents the vehicle velocity, E represents the grade of the slope, and W represents the vehicle weight.
The purpose of the dynamometer is to impose those forces on the vehicle which the vehicle would incur during actual operation on a road. Dynamometers for 2 WD vehicles (front or rear axle drive) include a roll (or a pair of rolls) for engaging the driven wheels of the vehicle being tested. Prior art dynamometers for 4 WD vehicles (front and rear axles coupled to the engine) generally include a roll or pair of rolls for supporting and engaging each pair of wheels (front and rear) with the rolls being mechanically connected so that all of the rolls rotate at the same speed.
Typically a power supplying and absorbing unit such as an electric motor (a.c. or d.c.) or a power absorbing unit such as a friction brake, eddy current brake or hydrokinetic brake is coupled to the roll or rolls for supplying power to and/or absorbing power from the roll(s) which in turn applies a retarding force to the surface of the vehicle wheels (e.g., tires) to simulate the road load forces. Inertial forces can be simulated by power supplying and absorbing units during both acceleration and deceleration but can be simulated by power absorbing units only during acceleration. Mechanical flywheels are generally used in conjunction with power supplying and/or absorbing units to simulate a part (or in some instances all) of the vehicle inertia. Vehicle velocity may be determined from the formula: ##EQU2## where V.sub.1 =the computed velocity at time t.sub.1, V.sub.o =the velocity at time t.sub.o, F=the measured force at the roll interface, I=the desired vehicle inertia and RL=the road load force. The implementation of this basic equation to control the operation of a dynamometer is explained in some detail in U.S. Pat. No. 4,161,116 assigned to the assignee of this application.
The rotational velocity of the roll is representative of V and can be accurately measured by coupling a speed encoder of the optical or magnetic pulse type to the dynamometer roll. However, there is no force measuring device which as a practical matter, can be placed between the rotating vehicle wheel and the roll. As a compromise, a force measuring device or transducer is generally placed either at the output of the power supplying and/or absorbing unit or between the flywheel assembly and the shaft connecting the flywheels to the roll. In either case, there are bearing friction and windage losses generated by the roll and/or flywheels which are not measured by the transducer. Such losses are commonly referred to as parasitic losses and must be compensated for in order to provide an accurate control signal for the power supplying and/or absorbing unit in the dynamometer.
A parasitic loss profile or curve of the lost force at the roll surface (due to parasitic losses) versus roll speed for the roll can be computed by measuring the force required to maintain the roll or rolls at several selected (e.g., three) speeds. Such a loss profile can also be calculated by using the actual inertia of the roll system and allowing the roll (or rolls) to coast down from a high speed while measuring the change of roll speed at selected points on the speed curve. A signal representative of the forces attributable to parasitic losses can then be added to the force signal measured by the transducer to provide a force signal representative of F.
This arrangement is satisfactory for testing 2 WD or 4 WD vehicles where all of the wheels engaging the rolls are positively driven by the vehicle engine. However, dynamometers for 2 WD vehicles cannot be used to test 4 WD vehicles for obvious reasons and prior art dynamometers designed for 4 WD vehicles cannot readily be used to test 2 WD vehicles because the measured force will include a force required to turn the non-driven wheels. This force, required to push or pull the passive wheels, is already included in the road load force with the result that the use of the above equation for controlling the dynamometer is compromised. The same problem exists in testing some 4 WD vehicles even on the prior art dynamometers designed for 4 WD provide a viscous vehicles. Many modern 4 WD vehicles provide a viscous coupling arrangement between one of the axles and the engine whereby power is supplied to the viscous coupled axle only when the two axles (front and rear) are turning at different speeds. Where all four wheels are turning at the same velocity e.g., on a test dynamometer, the force required to rotate the wheels on the viscous coupled axle will be included twice i.e., in the measured force F and RL.
There is a need for a versatile dynamometer apparatus and method for testing both 2 WD vehicles and 4 WD vehicles with various engine/axle coupling arrangements.