The Effect of Intake Temperature in a Turbocharged Multi Cylinder Engine operating in HCCI mode

Abstract

The operating range in HCCI mode is limited by the excessive pressure rise rate and therefore high combustion induced noise. The HCCI range can be extended with turbocharging which enables increased dilution of the charge and thus a reduction of combustion noise. When the engine is turbocharged the intake charge will have a high temperature at increased boost pressure and can then be regulated in a cooling circuit. Limitations and benefits are examed at 2250 rpm and 400 kPa indicated mean effective pressure. It is shown that combustion stability, combustion noise and engine efficiency have to be balanced since they have optimums at different intake temperatures and combustion timings. The span for combustion timings with high combustion stability is narrower at some intake temperatures and the usage of external EGR can improve the combustion stability. It is found that the standard deviation of combustion timing is a useful tool for evaluating cycle to cycle variations. One of the benefits with HCCI is the low pumping losses, but when load and boost pressure is increased there is an increase in pumping losses when using negative valve overlap. The pumping losses can then be circumvented to some extent with a low intake temperature or EGR, leading to more beneficial valve timings at high load.

Numerical simulation of cyclic variability in reactivity-controlled compression ignition combustion with a focus on the initial temperature at intake valve closing

“…29,30 Third, the compression ignition process is dominantly controlled by the chemical kinetics of the fuel, especially via the intake temperature, which plays an important role on cyclic variations in CA50. 12,25,31 Fourth, as Saxena and Bedoya 32 concluded, cyclic variations in HCCI combustion are mainly produced by fluctuations in the charge temperature and composition, and combustion is more sensitive to small variations in the compressed gas temperature. Perturbations of the compressed gas temperature can be induced by variations in the temperature and composition of the intake charge, residual gas, and EGR gas, as well as heat transfer losses and cooling of fuel injected in the intake port for RCCI engines.…”

“…The relatively independent behavior among the cyclic variations in different parameters has also been observed in experiments of HCCI combustion. Previous experiments 12,25,26 indicated that it is effective to utilize the cyclic variations in ignition timing to control the overall stability of HCCI combustion. Since the cyclic variations in CA50 dramatically affect the emissions, noise, and variability of RCCI combustion, 18 this study focused on the cyclic variations in ignition timing using the STD of CA50 as the metric.…”

HCCI Heat Release Data for Combustion Simulation, Based on Results from a Turbocharged Multi Cylinder Engine

“…With NVO the intake temperature directly affects how much internal EGR is needed for a given combustion timing and therefore the in-cylinder relative burn gas fraction. By increasing the intake temperature, less internal EGR is needed which lead to increased mass flow that can lead to higher boost pressure [12]. In addition to the internal EGR, external cooled EGR can be used and it was found that it could improve combustion efficiency while there seem to be low influence on the combustion duration [13] in HCCI mode.…”

“…The CA50 timing will affect the engine performance substantially and need to be controlled exactly to run the HCCI engine successfully. If engine speed goes up or if load is increased the CA50 window for stable engine operating will be narrowed [12], making the engine control more challenging.…”

“…The combustion timing and heat release rate has to be predicted together appropriate valve timings. If the engine is operated with NVO and short duration valve timings there can be high throttling losses over the valves [12] and this need to be predicted when simulating the engine performance. Since the boost pressure is used to decrease the combustion noise it is vital that the turbocharger performance is predicted accurately.…”