Heat Release Control in a Gas Engine by a Dual Flame Ignition
Shinsuke ONO*, and Yoshinori HIRAYAMA**
If the combustion is achieved in several zones in the combustion space, and the ignition of each zone is independently controlled by either the multiple direct injections coupled with an individual ignition or the multiple ignitions, the process may positively be changed during combustion period.
The experimental results obtained from changing fuel supply conditions by making use of the single cylinder test engine, show the widely changed heat release processes depending on the above control methods. And the possibility to bring a preferable combustion could be shown.
The numerical simulations of the cycle performance related to the combustion processes are carried out using the numerical simulator developed for the purpose of the present case, involving the multi-zone combustion models, and show that the optimum processes under these combustion control may exist.
Key Words: Gas engine, Heat release rate, Premixed gas, Fuel injection, Simulation, NOx, Combustion zone model
The performance of a reciprocating internal combustion engine depends greatly on combustion processes, when the specifications of an engine are fixed. A gas engine usually adopts a pre-mixed combustion and/or stratified charge combustion in the case of gaseous fuels. For the low density of gaseous fuel, combustion control through a fuel injection rate is not practical. Therefore, in order actively to control combustion processes in an engine cylinder, formation of a several distributed burning zones may be one of the solutions. In the present study, the combustion process or the heat release process in the engine cycle is controlled by the timings of multi-zone ignition in a combustible charge. Active heat release control by such a scheme is possible either in the following forms,
1) multi-ignition in a premixed charge, 2) multi-ignition in a stratified charge formed by a direct injection in a cylinder, 3) combination of two methods .
In the present study, the above methods were applied for the test engine with single cylinder. Depending on the ignition timing control, the indicated mean effective pressure and the emission characteristics of NOx widely changed via active control of heat release process .
These performances were examined by making use of the cycle simulation code designed for the present purpose .
A prediction of the cycle performance was made with Wiebe's function approaching the actual heat release rate . The thermodynamic state of the working gas during combustion was assumed, using the following zone models, respectively. (a) Single zone model (mixing state of a burnt and an unburned zone in the combustion space) (b) 3-zone model (state of the unburned zone, the burnt zone, and the reacting zone are separately evaluated) (c) multi-zone model (the burnt zone is divided into the finite multiple zones depending on the time of combustion).
2.THE SIMULATION OF THE CYCLE
The cylinder cycle is consisted of four processes; compression, combustion, expansion and intake-exhaust process. In this model, a p-v path associated with the combustion process changes according to the heat release rate. Cylinder space is divided into unburned and burned zones during the combustion process in order to estimate temperatures more precisely.
The thermodynamic analyses of each process of the model cycle are made with the following assumptions. The detailed calculation procedures are discussed in section 3 and 4.
1. The working substance is a mixture of ideal gases.
2. A composition of working substance changes with a process of the combustion reaction.
3. Each component gas remains associated.
4. The same amount of the released heat by combustion reaction is applied to the working substance to give a pressure rise according to the first law of thermodynamics.
5. A heat release rate of combustion follows the rate function of applied by Wiebe.
6. A heat loss from the combustion chamber is estimated by using Eicherberg's formula for the heat transfer coefficient.
7. A combustion temperature of burning zone is given from the adiabatic combustion temperature at the thermal equilibrium state.
8. The working substance in unburned and burned zones are adiabatically compressed and/or expanded.
*,** Kyushu University, Graduate school of Engineering
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