Comparing the Effect of a Swirl Flap and Asymmetric Inlet Valve Opening on a Light Duty Diesel Engine

Abstract

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 NO x 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 NO x 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 NO x 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 NO x emissions, by a combination of high levels of swirl and a reduced effective compression ratio.

Evaluation of exhaust gas recirculation techniques on a high-speed direct injection diesel engine using first law analysis

“…It is well understood from literature 10,11 that the throttling effect of closing a swirl flap will increase pumping work and reduce brake efficiency as indicated in Figures 5 and 6. It is also expected that efficiency will be affected by swirl flap induced changes to the mixing, combustion and heat transfer characteristics of the engine.…”

“…Closing the intake port throttle, hereafter described as the swirl flap, increases swirl levels in the cylinder as documented in a previous work. 11 A schematic diagram of the single cylinder engine along with the engine instrumentation measurement points is given in Figure 1.…”

“…10,11 However, excessive levels of swirl can reduce thermal efficiency due to increased convection losses, slowing combustion and increasing CO and THC emissions. 9,11–13…”

“…Increasing swirl enhances local fuel-air mixing leading to improved combustion efficiency and emissions oxidation 9 -albeit with potential penalties in NO x emissions and volumetric efficiency. 10,11 However, excessive levels of swirl can reduce thermal efficiency due to increased convection losses, slowing combustion and increasing CO and THC emissions. 9,[11][12][13] In this study, a detailed first law analysis (FLA) modelling tool has been developed, which has been used to assess the energy flows and efficiency of three different cooled high-pressure (HP) EGR methods at two different operating conditions (1500 rpm/6.8 bar nIMEP and 1750 rpm/13.5 bar nIMEP).…”

Effects of intake-port throttling on combustion behaviour in diesel low-temperature combustion

“…Figure 4), for the Rs = 2.43 test point is an increase in wall heat transfer rate and more effective convection of heat away from the evaporating and reacting fuel-air mixture in the piston bowl due to increased swirl motion. 22 Figure 6 details the variation of smoke, HC, and CO emissions with swirl number. The smoke emissions are uniformly low at this condition (FSN = 0.18, 0.14, and Figure 6.…”