Engine Testing Dynamometers and Fuel Mixture Basics

Electric Brake Dynamometer

The electric brake dynamometer transforms energy absorbed from the engine into electrical energy, unlike hydraulic brakes which convert it into heat. It consists of a stator (or inductor) that can pivot on a support resting on the bench via ball bearings, allowing for damping fluctuations around a shaft attached to the rotor (or armature) of the dynamo. The housing is mounted on a lever arm that collects the load weight, which determines the resistance torque measured on the dynamometer. On the opposite side of the housing, an extension of the lever arm carries a counterweight to balance the machine at rest.

Prony Brake: A Historical Perspective

The Prony brake is the simplest and most rudimentary dynamometer, and it was the first type used for engine testing. It consists of two wooden or fiber pads that embrace a circular flywheel. These pads are joined by adjustable straps for tightening. The adjustable force of springs applies normal force (N) on the jaws to tighten the flywheel. One of the pads is attached to a lever arm, from one end of which hangs a dish where weights are placed for measurement. At the other end, a counterweight is placed to balance the device before testing. Its biggest drawback is its significant inaccuracy and lack of precision.

Hydraulic Brake Dynamometer

The hydraulic brake dynamometer operates by absorbing energy transmitted from the engine. This absorption is achieved by a stream of water driven in a vortex ring by a turbine integral with the drive shaft. This water is then thrown against adjustable vanes located within the housing. The energy is transformed into heat due to the water's resistance to the engine's rotation, and this heat is dissipated by the constant circulation of water within the turbine. Hydraulic brakes are commonly used for factory testing of engines.

Understanding Degree of Dilution

The degree of dilution refers to the relationship between the weight of air in a mixture and the weight of air required for its stoichiometric ratio. For calculating engine power, the theoretical degree of dilution (λ) is assumed to equal unity (λ = 1). This is determined by the weight of air necessary to form the theoretical or stoichiometric mixture. This mixture is considered under an atmospheric pressure of 760 mm Hg, which corresponds to an air density of 1.293 kgf/m³. Therefore, the weight of air that forms part of the theoretical mixture is derived from approximately 12,000 liters of air per liter of fuel.