Design of a New Overhead Two-Stroke Engine Braking Device and Braking Performance Evaluation

The braking safety of heavy-duty vehicles is widely concerned. This paper proposed a new purely mechanical transmitted OHC two-stroke braking device. The rigid–flexible coupled dynamics model of the device and the engine working process simulation model were used for joint simulation. The effects of CR lift, environmental conditions, compression ratio, and braking type on the engine braking performance were comprehensively evaluated. The result shows: good consistency of valve operation is obtained by using pure mechanical transmission. During the braking process, the in-cylinder pressure acts directly on the valves and significantly affects the maximum valve lift of the CR phase, therefore excessive in-cylinder pressure will reduce the reliability of the braking device. When the CR lift increases from 1.9 to 2.8 mm, the braking power per liter increases at low altitude, but first increases and then decreases at high altitude. The decrease in engine speed and compression ratio as well as the increase in altitude lead to the decrease in braking power per liter. A benchmark of braking performance was taken from a high-compression-ratio (17) engine with a four-stroke decompression brake. For the high-compression-ratio (17) engine with two-stroke brake, the maximum in-cylinder pressure was reduced from 6.30 to 3.84 MPa, and the braking power per liter was increased by 24.5%–32.3% at low altitude and 93.2%–110.2% at high altitude. For the low-compression-ratio (11.6) engine with two-stroke brake, the maximum in-cylinder pressure is reduced to 2.83 MPa, and the braking power per liter changes less at low-altitude conditions and increases by 54.8%–67.1% at high-altitude conditions. The two-stroke braking device will greatly improve the braking safety and device components’ reliability on heavy-duty trucks, broaden the operating speed range, and promote the popularization and application of low-compression-ratio engines such as natural gas engines and methanol engines.