Reactivity Controlled Compression Ignition
Reactivity Controlled Compression Ignition (RCCI) is a combustion concept that can be considered as a hybrid between Spark Ignition (SI) and Conventional Diesel Combustion (CDC). This combustion concept is the technically viable alternative to Homogeneous Charge Compression Ignition (HCCI). Both principle achieve a very high efficiency with relatively low emissions, preventing the need for exhaust gas aftertreatment. The DOCAT 3500AR retrofitted diesel engine runs RCCI to provide the best possible performance while complying to Stage V emission standards.
RCCI is always operated with at least two different fuels with different reactivity, one high octane fuel and one low octane fuel. The low octane fuel is typically direct injected while the high octane fuel is port injected. The direct injection is timed in the compression stroke in such a way that the low octane fuel has enough time to mix to a certain degree with the high octane fuel and air. This mixing happens in a rather homogeneous manner, where the injection timing determines the amount of fuel stratification in the cylinder.
This combustion process uses auto-ignition just like CDC engines. However, at the moment of ignition the charge is rather homogeneous, like in SI engines. The timing of ignition is determined by the stratification of the fuel and the reactivity of the fuel, which is determined by the ratio between the high and the low octane fuel, hence the name Reactivity Controlled Compression Ignition.
The main reason why RCCI is exceptionally efficient is that the heat release of the combustion has a high degree of controllability, which means that the thermodynamic efficiency can be fully optimized. To damp the chemical reactions we operate the engine in lean burn which also reduces the heat losses. When running CDC a lot of the heat is transferred to the cylinder walls because of high peak temperatures of burning diesel sprays. RCCI operates with a more homogeneous mixture resulting in a more homogeneous temperatures distribution in the cylinder, which leads to lower heat losses. ArenaRed achieved an efficiency of 51.5% on the DOCAT 3500AR engine. According to research, an engine optimized for RCCI is able to achieve an efficiency of 58%.
The lean and homogeneous burn when using RCCI is the main reason the peak temperatures can be kept low, which result in very low NOx emissions. The premixed lean combustion of RCCI results in an exceptionally low soot emission. The high controllability gives ArenaRed the ability to control the peak cycle temperature which determines the Carbon Monoxide (CO) and Methane (CH 4) emissions. ArenaRed achieved Stage V compliant emissions on the DOCAT 3500AR engine without exhaust gas aftertreatment. According to researcher, an engine optimized for RCCI can achieve EURO6 emission standards without exhaust gas aftertreatment.
Single Fuel RCCI
RCCI has to be operated with two different fuels, one high octane fuel and one low octane fuel. In automobile applications this can be seen as cumbersome and can discourage people to choose for an engine running RCCI. A solution is that for a high octane fuel is used for both injections, but that a cetane-booster is added to the direct injected fuel to increase reactivity. The added cetane-booster requires a refill of 5 liters every oil change, which could be done by the car mechanic. In this way the car owner has all the benefits of owning a vehicle operating RCCI without having to worry about fueling two different fuels.
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