Two-stroke future for heavy-duty engine braking

  • 03-May-2016 11:33 EDT
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Jacobs’ HPD compression release engine brake adds intake valves and turbo boost control to hike efficiency.

It is thankfully rare that the driver of a heavy truck or bus loses control of the vehicle because of brake failure caused by overheating the foundation brakes. This is partly because of the widespread use of auxiliary braking systems, which use equipment other than the foundation brakes to reduce speed, particularly on long downhill grades.

The compression release brake provides an alternative to traditional exhaust brakes and retarders, with a design pioneered by U.S.-based Jacobs Vehicle Systems in the 1960s. Conventionally, it takes the backpressure retardation principal a step further by controlling the exhaust valve opening on the exhaust stroke to create compression and therefore resistance to motion rather than the usual scavenging. Since the fuel supply is turned off when there is no demand for fuel in a modern diesel engine, there is no interference with the normal functioning of the engine. The exhaust valve remains closed until top dead center.

The effect is produced by installing a bridge under the cam follower. This contains a control solenoid valve that controls the flow of oil to a hydraulic actuator, which acts on the cam instead of the regular cam follower. The oil supply is drawn through a drilling in the rocker shaft. Under power, the solenoid valve is closed and the actuator piston is locked to the bridge enabling the valve to follow the cam profile for four-stroke operation. When braking is required, the solenoid valve is opened, unlocking the actuator from the bridge and causing the valve to remain closed until top dead center is reached. Since the cam profile of the engine remains the same and the bridge cannot extend valve lift or timing, the brake does not pose a threat to the integrity of the engine.

It follows that larger-capacity engines offer the potential for more retardation, but the trend in recent years has been toward engine downsizing, which has had the effect of reducing engine braking potential. At the same time, rolling resistance has been reduced by the introduction of more aerodynamic vehicle designs, low rolling resistance tires, and reduced driveline friction. In other words, there is less built-in resistance to motion in a modern heavy vehicle. At the same time, gross vehicle weights for heavy trucks have increased since the 1990s in regions such as Europe, with maximum permissible weight reaching 44-tonnes for regular use in the European Union.

Jacobs’ data suggests that continuous improvements in engine design and engine brake design have helped to raise compression release engine braking from around 8.0 kW/L at 1500 rpm from a 1960s heavy diesel with mechanical fuel injection to 20 kW/L at 1500 rpm by the late 1990s, when engines had adopted dual camshaft designs and the engine brake had been integrated into the valve train with a dedicated cam.

High Power Density engine brake

Despite the greater efficiencies of engine brake designs, increasing gross weights, lower rolling resistance, and engine downsizing have driven the need for more auxiliary braking power. Jacobs response is the High Power Density (HPD) engine brake.

With HPD, the same system of bridge, control valve, and hydraulic actuator is applied to the inlet cam as well. This enables the engine to switch from four-stroke operation under power to two-stroke operation under braking, thereby doubling the number of “braking” strokes. At the same time Jacobs is able to use boost from the turbocharger more effectively by controlling it to optimize boost charging for braking.

“You need to look at this as a system of how we control the valves on the intake, the exhaust, and the turbocharger,” explained Tom Howell, Director of New Technology at Jacobs Vehicle Systems. “All these components need to be considered. By having two intake events, it enables us to get a larger amount of airflow through the engine and by having two compression release events, that’s obviously utilizing the air that’s provided by the two intake events.”

The result has been to increase the available braking power to around 28 kW/L at 1500 rpm, with a maximum of around 37 kW/L at 2200 rpm. Jacobs claims one and a half times the braking performance of a traditional compression release brake over the engine’s operating range with the same retardation at 1400 rpm as at 2100 rpm with the previous system. For a 13-L engine, Jacobs claims 2000 N·m (1475 lb·ft) of retarding torque at 1300 rpm and above with 611 kW (819 hp) of braking power at 2500 rpm.

As fitted to the Mercedes-Benz Actros demonstration vehicle, Jacobs claims braking power of around 370 kW (495 hp) at 1500 rpm. Jacobs says that the total system weight is around 12 kg (26 lb) compared with 150 kg (330 lb) for a hydraulic retarder. All additional heat is expelled through the exhaust system, helping to keep diesel particulate filters at working temperature.

Jacobs provided two identical Mercedes-Benz Actros models loaded to 40-tonnes gross combination weight for demonstration, fitted with 13-L in-line six-cylinder engines. One was equipped with the conventional compression release brake and the other with the HPD system. Demonstration drives took place on the Hill Route at the Millbrook Proving Ground in the U.K., not normally open to laden trucks because of the 26% steep descent on one section.

While we were able to control descent on the 26% gradient in the vehicle equipped with the conventional system, additional check braking using the foundation brakes was needed. The HPD-equipped vehicle gave far greater control and with HPD engaged early enough, it enabled descent without using the foundation brakes, notable on such a steep descent.

The HPD-equipped vehicle was equipped with a potentiometer to control the turbocharger boost because it provided too much retardation on lesser grades. This could take the form of a four or five position column stalk on an OE-integrated installation.

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