Comparing the Effect of a Swirl Flap and Asymmetric Inlet Valve Opening on a Light Duty Diesel Engine 2017-01-2429
Diesel engine designers often use swirl flaps to increase air motion in cylinder at low engine speeds, where lower piston velocities reduce natural in-cylinder swirl. Such in-cylinder motion reduces smoke and CO emissions by improved fuel-air mixing. However, swirl flaps, acting like a throttle on a gasoline engine, create an additional pressure drop in the inlet manifold and thereby increase pumping work and fuel consumption. In addition, by increasing the fuel-air mixing in cylinder the combustion duration is shortened and the combustion temperature is increased; this has the effect of increasing NOx emissions. Typically, EGR rates are correspondingly increased to mitigate this effect. Late inlet valve closure, which reduces an engine’s effective compression ratio, has been shown to provide an alternative method of reducing NOx emissions. Recently introduced technologies combine these two effects by retarding only the swirl port valve, increasing in-cylinder swirl while simultaneously reducing the effective compression ratio.
In this paper the effects of using a swirl flap and offset cams are compared. Four different swirl flap positions (ranging from fully open to fully closed) were investigated using standard cams and valve timings. Results were compared with the engine’s operation when using two offset cams providing two different levels of retard on the swirl port-30 and 60 crank angle degrees (CAD) respectively. Engine emissions, fuel consumption, and combustion parameters were measured and compared in order to elucidate the effects of phased cam operation.
The results show that the use of a cam retarding the opening of the swirl port can reduce NOx emissions at certain speed/load conditions without adversely affecting other emissions. In addition significantly retarding the swirl port closure can reduce FSN emissions to near zero with low NOx emissions, by a combination of high levels of swirl and a reduced effective compression ratio.