A novel staged internal combustion engine

This Isolated Combustion And Diluted Expansion (ICADE) engine0 achieves combustion chamber isolation without using an externally controlled valve. Typically 100% excess air is provided in the form of dilution air which has been compressed separately to double the combustion air ratio. The added dilution air compression ratio in the ICADE mode increases the ideal OTTO and DIESEL type thermal cycle efficiencies up to 15%. The ICADE geometry design equations have been presented here. Three reference stoichiometric burning CNG engines have been reconfigured on paper to operate in the ICADE mode. This results in a combustion chamber clearance to diameter ratio improvement by up to 500%. An important feature of ICADE is that the volumetric changes during combustion are reduced by a factor of two. This feature improves the ICADE thermal efficiency by up to 8% over that of a stratified charge engine with the same equivalence ratio, and up to 20% relative to the stoichiometric reference CNG engines. The addition of a ball type check permits earlier quenching but its effect on the thermal efficiency is limited to a few percent, its main benefit is to reduce NO,. *Professor, Mechanical and Aerospace Engineering West ~irginia University, Associate Fellow AIAA **Graduate Research Assistant, Mechanical and Aerospace Engineering, West Virginia University, member AIAA *+*Assistant Professor, Aeronautical and Astronautical Engineering, University of Illinois at Urbana-Champaign, member AIAA O US Patent # 5,239,959, Aug. 31, 1993 Introduction The thermal efficiency of the spark ignited internal combustion engine is limited to about half the value of that given by the Otto cycle at the same compression ratio. For example at r,=10, find qm,=60% in the cold air cycle with specific heat ratio y=1.4. The actual engine efficiency is about half as much. The main reasons for this difference are: 1) The increase in gas specific heat with temperature reduces the specific heat ratio. For example at y=1.3 find tlotto=50% . 2 ) The cylinder wall heat losses are not accounted for in the Otto cycle 3) Finite flame speed limits the combustion rate, and results in volumetric changes during heat release. This increases the compression work and lowers combustion pressure rise and potential for expansion work. Note the ideal Otto cycle analysis is based on instantaneous heat release at TDC. Much research effort has been spend to alleviate some of the above mentioned shortcomings in an actual engine. 1) Using lean mixtures, lowers the peak temperature and increases the average specific heat ratio. To compensate for the associated reduced flame speed, some engines use a stratified charge with a less lean mixture near the ignition source. 2 ) To reduce cylinder wall heat loss and improve flame speed, the combustion chamber clearance to diameter ratio is increased so as to minimize the wall area to volume ratio. Ceramic lined combustion chambers have been used in an attempt to create an adiabatic IC engine. This technique has limited success as it creates almost as many problems as it solves. 3) To reduce the volumetric changes during combustion at a finite flame speed, many variable stroke engines have been proposed1. These provide more time with the piston near top dead center and thus minimize the efficiency loss associated with compression and expansion during heat release. All these attempts have had limited success due to the complexity and high friction loss associated with a variable stroke crank mechanism. To further improve m the efficiency of the stratified charge engine and to provide the pressure differential needed for rapid mixing of the combustion products with the dilution air, the combustion chamber can be mechanically isolated from the dilution air. This technique allows a near constant volume combustion and an improved combustion chamber geometry. Many patents have been issued on various isolated combustion chamber engine configurations. For example see Ref. 2 and 3, few of these ideas have materialized because of problems associated with the externally activated isolation valve, and its seal in the valve guide. The Isolated Combustion and Diluted Expansion Engine Concept The above mentioned limitations of the conventional IC engine can be improved upon by modifying the engine configuration to operate in the ICADE mode. This stands for Isolated Combustion and Diluted Expansion. A SPE (Small Piston Extension) is mounted on top of the piston. The SPE can have any shape like a pie shaped section of the piston upper surface or a small cylinder, as shown in Fig. 1. The SPE enters a corresponding cavity, hollowed out in the cylinder head, at a specified angle (6,) BTDC, for example 30 BTDC. The crank angle (e,) , at which the SPE isolates the combustion air should be selected so as to provide adequate time for direct cylinder fuel injection, mixing and spark advance in case the fuel is not compression ignited. The SPE has four functions. First as the piston motion actuates the SPE it serves as an isolation valve, thus eliminating the need for an externally activated isolation valve with its valve stem leakage problem. Second it permits the dilution air which remains inside the cylinder, to be compressed about twice as much as the combustion air for improved thermal efficiency. Third the combustion air is compressed more than usual before reaching the isolation crank angle and much less after isolation, thereby simulating the near constant volume combustion of a variable stroke engine. Fourth the combustion chamber clearance to diameter ratio is increased up to five times to reduce the flame front traveling distance and thus combustion crank angle. For the ICADE configuration shown in Fig. 1, the combustion chamber isolation crank angle (6,) before TDC is the same as its opening angle ATDC. Because the combustion process is usually completed sooner, there are some advantages to adding a ball type check valve to the SPE, as shown in Fig. 2. Such a ball valve allows early quenching of the combustion products which minimizes the formation of NO, and slightly improves the thermal efficiency. The piston motion centripetal force, assisted by the added compression ratio (rd) of the dilution air, automatically closes the ball valve prior to reaching the isolation crank angle (8,) BTDC. The combustion generated excess pressure is used to force the ball valve open against the acceleration force which tries to keep it closed. It is important to point out that most advantages of the ICADE configuration are already obtained without the ball type check valve shown in Fig. 1. Thus the added complexity of using a ball valve may not be justified. Using a theoretical cold air standard cycle Otto type analysis with instantaneous heat addition, the ICADE configuration with a ball type check valve which opens at TDC, , would produce an unusual pressure/crank angle diagram a shown in Fig. 3. After point (i,) the combustion chamber pressure rises only slightly whereas the dilution air pressure rises sharply. The corresponding cycle thermal efficiency can be written in a general notation using a factor (B) . The has air value of B,, differs for each cycle and been shown in Ref. 4 for the ideal cold standard Otto, Diesel and ICADE cycles. Where T, is the maximum to minimum temperature ratio and r, is the volumetric combustion air compression ratio. For the ICADE cycle there are an additional three independent design parameters which control the ICADE engine geometry, they