Method of operating a dual fuel internal

Details Method of operating a dual fuel internal Method of operating a dual fuel internal

2001-02-27

2003-04-22

Yuen, Henry C. (Department: 3747)

Internal-combustion engines

Burning by highly compressed air

Oil engine air preheated

C123S525000, C044S300000, C585S014000, C208S015000

Reexamination Certificate

active

06550430

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention provides an improved method of operating a dual fuel internal combustion engine, especially where the engine is required to run at high speed, for example at over 3,000 rpm. The fuels used include one high octane fuel such as gasoline, and one fuel of high cetane value. The engine is preferably a conventional reciprocating engine of the piston in cylinder type.
2. Prior Art
Traditionally there have been four primary modes of operation for reciprocating internal combustion engines: spark ignition (SI), homogeneous charge compression ignition (HCCI), compression ignition (CI) and dual fuel compression ignition (DFCI). SI and CI engines have been commercially dominant due to the more simplistic and inexpensive control systems required for satisfactory operation.
SI Engine
Spark ignition (SI) engine operation involves ignition of a homogeneous or stratified mixture of air and readily vaporized high octane fuel, such as gasoline, using an electrical discharge (spark) from one or more ignition devices such as a sparkplug, located in the combustion chamber of the engine. The fuel, which may be in a gaseous or atomized/vaporized liquid form, may be entrained with the air drawn into the cylinder of the engine, using a carburetor or fuel injector located in the intake air system. Alternately, the fuel may be injected directly into the cylinder through a fuel injector located in the cylinder or cylinder head. Engine load and speed control is primarily accomplished by controlling the quantity of air which enters the cylinder and remains in the cylinder during the compression process, just prior to ignition of the air/fuel mixture. The fuel quantity is reduced approximately proportional to the air quantity to provide a combustible mixture. The combustible mixture ranges between slightly richer than stoichiometric at high loads to no more than 35% leaner than stoichiometric at low to moderate loads. Throttling of the air or air/fuel mixture at low loads reduces the quantity of air/fuel mixture drawn into the combustion chamber, thereby reducing compression pressures and correspondingly reducing thermal efficiency and fuel efficiency. In addition, throttling increases the resistance of air flow into the combustion chamber generating a parasitic loss in engine power, further reducing overall thermal efficiency and fuel efficiency.
Ignition and combustion of the air/fuel mixture in SI engines is relatively slow, particularly at low loads, resulting in less than optimal thermal efficiency and fuel efficiency since only a portion of the fuel's energy is released at the point of maximum compression. Combustion of the air/fuel mixture begins at the sparkplug (under normal operating conditions). Since the flame has a single flame front, a finite period of time, which is dependent on many factors, is required for the flame (generated by the spark at the sparkplug) to propagate across the combustion chamber. The air/fuel mixture furthest from the sparkplug is ignited substantially later than the air/fuel mixture near the sparkplug. During flame propagation the pressure in the combustion chamber increases. The compressed air/fuel mixture furthest from the flame front is compressed to higher and higher values awaiting the flame. If the compression pressure and corresponding temperature of the air/fuel mixture awaiting the flame is sufficient, as well as the exposure time, the air/fuel mixture will autoignite before the flame reaches it. Autoignition of the air/fuel mixture results in very rapid rates of combustion generating high combustion pressures, rates of combustion pressure rise and combustion knock, which may cause engine damage depending on many factors. The octane rating of a fuel is a measure of the fuel's resistance to autoignition and combustion knock, with higher octane values indicating greater autoignition resistance. SI engines employ high octane fuels to minimize autoignition of the air/fuel mixture.
SI engine thermal efficiency and fuel efficiency is less than optimal due to compression pressure constraints dictated by many factors including fuel octane, throttling of the air or air/fuel mixture at low loads which results in increased parasitic losses in engine power and the relatively slow combustion process. Although the combustion process is relatively slow, increases in engine speed generate increased turbulence within the combustion chamber. The increased turbulence accelerates propagation of the flame front and the combustion process such that combustion efficiency is maintained even at high engine speeds.
HCCI Engines
Homogeneous charge combustion ignition (HCCI) engines operate similarly to SI engines in that a homogeneous or partially stratified mixture of air and high octane fuel such as gasoline or natural gas is combusted. However, ignition of the air/fuel mixture is not accomplished using a spark. Ignition is accomplished by compressing the air/fuel mixture to a high degree such that instantaneous autoignition of the air/fuel mixture occurs throughout the combustion chamber nearly simultaneously. The fuel, which may be in a gaseous or atomized/vaporized liquid form, may be entrained with the air drawn into the cylinder of the engine, using a fuel injector located in the intake air system. Alternately, the fuel may be injected directly into the cylinder through a fuel injector located in the cylinder or cylinder head. Engine load and speed control is primarily accomplished by controlling the quantity of fuel which enters the cylinder and remains in the cylinder during the compression process, just prior to ignition of the air/fuel mixture. The quantity of air supplied to the engine is not throttled to control engine load and speed as is done with SI engines.
HCCI engines were developed to provide greater thermal efficiency and fuel efficiency than SI engines, as well as to reduce low load emissions of oxides of nitrogen (NOx). HCCI engines can operate at higher compression pressures prior to combustion than SI engines, for the same fuel octane value since, unlike with SI engines, autoignition of the air/fuel mixture is desired. In addition, the autoignition process causes very rapid combustion of the air/fuel mixture. The HCCI combustion process generates higher thermal efficiency and fuel efficiency than the SI combustion process, due to higher compression of the air/fuel mixture and a very short duration combustion process whereby most of the energy of the fuel is released at or near the point of maximum compression. In addition, less energy is required to induct air into the engine at low loads since the combustion air is not throttled to control load as is done with SI engines.
However, due to the very short combustion duration and high degree of air/fuel mixture compression, excessive combustion pressures and rates of combustion pressure rise are generated at high loads with stoichiometric or near stoichiometric air/fuel mixtures. As such, HCCI engines are typically operated at low to moderate loads with lean air/fuel mixtures, thereby reducing combustion pressures by reducing the quantity of heat released and by increasing the combustion duration slightly. The lean air/fuel mixtures also tend to decrease NOx emissions relative to SI engines at equivalent loads, due to reduced peak combustion temperatures. However, the lean air/fuel mixtures required to provide low loads tend to generate high levels of unburned hydrocarbons, typically referred to as total hydrocarbon (THC) emissions, due to incomplete combustion of the fuel. At low loads incomplete combustion is caused by low combustion temperatures and is further exacerbated by the high resistance to ignition of the high octane fuel and lean air/fuel mixture.
In addition to load limitations, difficulties are encountered with HCCI engines with regard to controlling the timing and intensity of the autoignition process for optimum operation. Autoignition timing and intensity is not controlled by a single factor, numerous factors in

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