Internal-combustion engines – Four-cycle – Having subcharger associated with the cylinder
Reexamination Certificate
1998-04-01
2002-09-03
Kamen, Noah P. (Department: 3747)
Internal-combustion engines
Four-cycle
Having subcharger associated with the cylinder
Reexamination Certificate
active
06443125
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a system for improving the fuel economy of vehicles, and more specifically to a system for improving the low-load efficiency of automotive spark-ignition four-stroke internal-combustion engines.
The maximum power capability of typical vehicle engines is significantly greater than the average power actually used while driving. On average, new cars sold today in the United States can deliver a maximum power of approximately 100 kilowatts. For most drivers, however, the average power actually used during city and highway driving is approximately 10 kilowatts. This can be appreciated from the stepped line in prior art
FIG. 1
, in which engine power output is shown on the abscissa, or horizontal, axis, and the percent of total driving time spent at a given engine power level for a typical driver and vehicle is read from the left ordinate axis. The low average power use is a problem, because conventional vehicle engines have poor efficiency when operated at light power levels. Since vehicle engines are operated most of the time at light power levels, fuel economy is poor. The efficiency of a typical prior art vehicle engine relative to engine power output is shown by the curved line in
FIG. 1
, in which brake engine efficiency is read from the right ordinate axis. As can be appreciated from
FIG. 1
, prior art engines are operated at low efficiency most of the time.
The problem of poor light load engine efficiency has been known for some time. In 1958 the Society of Automotive Engineers (SAE) published “Determination of True Engine Friction,” SAE Trans., Vol. 66, pp. 649-661 which deals with engine friction losses, which are an important cause of poor light-load engine efficiency. Engine friction losses, which include mechanical sliding friction and aerodynamic pumping losses have only marginally improved since 1958. Today, typical new midsize passenger cars have an engine efficiency of approximately 18.2% during urban driving and 25.6% during highway driving, according to the 1994 Program Plan of Partnership for a New Generation of Vehicles (PNGV), a United States government and industry partnership including the major U.S. automobile manufacturers.
The PNGV was formed in 1993 to attempt development within ten years of one or more production prototype vehicles having a fuel economy of up to three times that of today's passenger cars. Methods of significantly increasing fuel economy are described in “The USDOE Propulsion Research and Development Program”,
Transportation and Global Climate Change,
1993, and in “Inventions Needed”, PNGV, March 1995. Fuel economy and exhaust emission levels can be improved with a hybrid power train. Hybrid vehicles are characterized as having one, typically very small, engine that efficiently generates for the vehicle the base load power requirement of approximately 10 kilowatts. In hybrid vehicles, the base load engine is unable to deliver the maximum power requirements of the vehicle. Consequently, a second engine or motor is employed to permit the hybrid system to deliver maximum power needs. Problems with hybrid power systems include the cost of two power sources, complexity and bulk.
FIG. 3
shows generally engine size relative to vehicle weight for various types of vehicles. Vehicle weight divided by engine cylinder displacement, W/D in units of kilograms per liter, is shown on the abscissa axis. Engine maximum power output divided by engine cylinder displacement P/D, in units of kilowatts per liter, is shown on the ordinate axis. A vehicle power to weight ratio of 0.055 kW/kg is represented by the line E, which indicates generally the lower bound of power and thus acceleration for vehicles sold today in the United States. Thus, this is considered to be the minimum ratio for providing suitable acceleration. Some very low power European city cars fall below line E. Race and exotic performance cars fall in area A of the diagram; conventional passenger cars are in area B; Japanese micro cars fall in area b; and hybrid vehicles described above fall in area C. All, or virtually all, passenger cars sold world wide and having a vehicle power to weight ratio greater than 0.055 kW/kg are in areas B and b. The curved line T is intended to represent the historical trade-off of power vs. efficiency for vehicle engines, from vehicles having high fuel economy in the lower right of the diagram, to vehicles having high performance in the upper left corner of the diagram. The area C, which generally represents the primary engines of hybrid vehicles, is below line E. Consequently, hybrid vehicles require a secondary engine in order to provide the power to weight ratio and acceleration of vehicles sold today in the United States. As stated above, this secondary power source adds significant cost, complexity and bulk to the vehicle.
FIG. 4
is similar to
FIG. 3
, except that it shows on the abscissa axis a ratio of compression ratio times vehicle weight to combustion chamber volume Cr(W/d), rather than vehicle weight to engine cylinder displacement, W/D. In general,
FIG. 4
relates to the combustion chamber volume rather than the cylinder displacement, and thus more precisely indicates automobile fuel economy than FIG.
3
. Cr is the maximum compression ratio of the engine and d is the minimum combustion chamber volume of the engine. All passenger cars shown in FIG.
3
and having a vehicle power to weight ratio greater than 0.055 kW/kg are shown in FIG.
4
. Dot H represents one hybrid vehicle, and is located below line e. As stated above, hybrid vehicles require a secondary engine in order to provide the power to weight ratio and acceleration of vehicles sold today in the United States. This secondary power source adds significant cost, complexity and bulk to the vehicle.
In a laboratory environment, direct injection diesel engines have demonstrated potential for attaining fuel efficiencies almost as high as those of hybrid power trains. While it may be possible to attain the PNGV mileage objectives with an advanced diesel engine, the exhaust emission levels of these diesel engines are unacceptable.
Another approach that has been attempted for improving non-hybrid power train engine efficiency is variable compression ratio. A variable compression ratio systems tested by Volkswagen is described in SAE paper No. 870610. Variable compression ratio systems with variable valve timing are described in U.S. Pat. No. 5,255,637 issued to M. M. Schechter, and in SAE paper No. 950089, February 1995, by M. M. Schechter et al., and in
Automobile Technology of the Future,
pp. 101-106, SAE, 1991. Variable compression ratio mechanism have been known for some time, and are described in SAE paper No. 640060, published in 1963, and SAE paper No. 901539 published in 1990. The general type of variable compression ratio mechanism used by Schechter was invented by the British Internal-Combustion Engine Research Institute (BICERI) in circa 1959. In prior art spark-ignition variable compression ratio engines, compression ratio is increased at light loads where the tendency of an engine to knock is at a minimum. Since thermodynamic efficiency increases with increasing compression ratio, one would expect increasing the compression ratio to increase light-load engine efficiency. However, the combustion chamber surface area to volume ratio also increases, which results in higher combustion chamber heat loss levels, which can be appreciated from prior art FIG.
10
. Prior art
FIG. 10
shows combustion chamber surface area on the vertical axis and engine power output at a certain engine speed on the horizontal axis. The surface area to combustion chamber volume ratio of typical vehicle engines is represented by cross hatched box
100
. The surface area to combustion chamber volume ratio of a variable compression ratio engine is represented by line
102
. As can be appreciated from
FIG. 10
, at light power levels, combustion chamber surface area increases significantly for these variable compression ratio engines, which
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