Source: Cycle World
Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)The spark ignition piston engines that power most motorcycles burn a mixture of a volatile liquid hydrocarbon and air. Mixture formation is the process of achieving an accurately proportioned, uniform, and easily ignitable mixture of air and fuel vapor.In combustion, the hydrogen and carbon atoms in the liquid fuel are allowed to combine with the 21 percent of atmospheric air that is oxygen, transforming higher-energy molecular structures into others of lower energy. The energy difference is released as heat.The word “hydrocarbon” tells the tale: The heat of combustion, expressed as molecular motion of much-increased velocity, knocks hydrocarbon molecules apart, allowing them to combine with oxygen to form di-hydrogen oxide (better known as water, or H2O) and carbon dioxide (the Bad Guy in the present global warming predicament).We’ve all seen water dripping from the tailpipes of vehicles whose engines have just been started. Water vapor in the exhaust gas—a product of combustion—condenses when it hits the cool metal of the exhaust system. Drip-drip it goes, puzzling interested seven-year-olds.In so-called “chemically correct” fuel-air combustion, the proportions of fuel to air are such as to produce only water and carbon dioxide—no left-over hydrogen, carbon, or oxygen. This is how the oxygen sensors of modern digital fuel injection systems operate by monitoring leftover oxygen).If there is too much fuel for the oxygen supplied, the mixture of fuel and air is said to be rich. If there is too much air for the fuel supplied, the mixture is termed lean. In either case, because the uncombined excess molecules carry away some heat but generate none, energy release is reduced and flame speed slows.Experiment has shown that an electric spark cannot reliably ignite fuel-air mixtures that are more than either 20 percent lean (approximately 18 parts air to one part fuel) or 20 percent rich (10 parts air to one fuel). Although there are now engines that use other means of ignition to burn mixtures leaner than 18-to-one, let’s stick to spark ignition for now.CarburetionThe earliest way to create an ignitable fuel-air mixture was by evaporation. The engine’s intake air was passed through layers of fuel-soaked cloth on its way to the cylinder. This was the wick carburetor. Because of the difficulty of controlling the mixture, it worked best at constant engine load.In 1893 Wilhelm Maybach patented the spray carburetor. It made use of the fact that moving air has less pressure than still air: The partial vacuum in fast-flowing intake air allowed the higher pressure in a nearby fuel reservoir (aka float bowl) to push fuel into that intake airstream.As air moves through the carburetor, fuel is pushed into the airstream. (Mark Hoyer/)In the collision of fuel with intake air moving at hundreds of feet per second, the fuel stream was broken up into particles held in roughly spherical form by the fuel’s surface tension (surface tension is what allows the insects known as “water striders” to walk on water). The pressure of the air upon hitting such droplets first flattens, then punches in their upstream faces. Surface tension then pulls them into the form of tiny rings whose wobbly instability causes them to break up into even smaller droplets. The higher the air velocity, the finer the droplet size.Droplet breakup increases the total fuel surface area exposed to the surrounding air, accelerating fuel evaporation.Fuel InjectionIn the port fuel injection employed on most motorcycle engines of today, the fuel injector is a solenoid-operated valve, usually with eight or 12 tiny holes through which fuel sprays. The droplet breakup process described above operates here as well, but is given a head start by the finer initial droplets produced by the multi-hole injector.Port fuel injection produces finer fuel droplets than a carburetor. (Yamaha/)Early single-orifice injectors tended to work less well than carburetors because many of the droplets they produced were quite large. At least one brand of early fuel-injected bike engine puzzled owners because engine oil level increased during use! Large unevaporated fuel droplets were able to reach the cylinder walls and be scraped down into the crankcase by the piston’s oil scraper ring, diluting the crankcase oil and gradually raising its level.In both carburetors and port fuel injection, fuel moves toward the engine in three forms: as evaporated fuel vapor, as some remaining droplets, and as “wall wash” (liquid fuel sliding along the inner walls of the intake duct).Direct InjectionA third technology of mixture formation is Gasoline Direct Injection (GDI), by which an injector operating at much higher fuel pressure (typically up to 2,200 psi) sprays fuel directly into the cylinder’s combustion chamber.TimeTime is an important variable in mixture formation. Because carburetors are located at some distance from the intake valve(s), they provide the longest “time-of-flight” during which fuel droplets and wall wash can evaporate. Port fuel injection of the kind presently most used on auto and motorcycle engines provides shorter time-of-flight, so for engines of high peak rpm a second set of “showerhead” injectors may be provided, located just above each intake pipe’s bellmouth. At higher revs, showerhead injectors take over most of the fueling, providing more time for droplet evaporation.Fuel evaporation can also be sped up by heating the fuel. Many snowmobiles were given carburetor fuel bowls heated by engine coolant. In F1 racing, Honda in the 1980s era of turbo engines running on highly antiknock but slow-evaporating toluene-based fuel were provided with fuel heaters. Carbureted auto engines for many years assisted fuel evaporation by use of exhaust-heated intake manifolds.In general, the harder a blender of racing fuel tries to increase its octane number (a measure of resistance to engine knock or ping), the slower-evaporating the fuel becomes.Another technique of assisting fuel evaporation was to point the injectors upstream. Not only did this increase the relative velocity of collision with the airflow, it also extended evaporation time.The shortest time of evaporation is provided by GDI, injecting into the combustion chamber itself. To compensate, injection pressure is increased. Higher injection pressure accomplishes two things:It can get the desired fuel charge into the cylinder quickly enough to leave adequate time for evaporation.It increases the velocity with which the fuel hits the air, thereby achieving smaller fuel droplet size.Direct injection is not a new technology, having been applied to German military aircraft piston engines in World War II, and in the mid-1950s to Mercedes racing cars. It was later (late 1980s to early ‘90s) developed in a form suitable for two-stroke engines as a means of greatly reducing their UHC exhaust emissions.It appears that once fuel droplets become 10 microns (0.00036 inch) or smaller they behave during combustion as if fully evaporated.Modern auto and motorcycle port fuel injection is not constant-flow. Each cylinder has its own solenoid-operated injection valve, located under the throttle butterfly. The valve is opened by sending current to the solenoid, and is closed by a spring when the current is switched off. Fuel delivery is controlled by how long the injector is held open each 720-degree cycle, called “on-time.” The accurate control over air-fuel mixture this makes possible is an essential technology in delivering high performance while meeting exhaust emissions limits. Older riders and drivers will remember the “stutter-and-stall” era of the 1970s and ‘80s, when lean carburetor jetting was required to meet those limits.There have been many constant-flow injection systems, most famous of which is Hilborn injection, used on Champ cars from the late 1940s to the 1970s, and remaining in use in specialized off-road applications to this day.
Full Text:
Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)
The spark ignition piston engines that power most motorcycles burn a mixture of a volatile liquid hydrocarbon and air. Mixture formation is the process of achieving an accurately proportioned, uniform, and easily ignitable mixture of air and fuel vapor.
In combustion, the hydrogen and carbon atoms in the liquid fuel are allowed to combine with the 21 percent of atmospheric air that is oxygen, transforming higher-energy molecular structures into others of lower energy. The energy difference is released as heat.
The word “hydrocarbon” tells the tale: The heat of combustion, expressed as molecular motion of much-increased velocity, knocks hydrocarbon molecules apart, allowing them to combine with oxygen to form di-hydrogen oxide (better known as water, or H2O) and carbon dioxide (the Bad Guy in the present global warming predicament).
We’ve all seen water dripping from the tailpipes of vehicles whose engines have just been started. Water vapor in the exhaust gas—a product of combustion—condenses when it hits the cool metal of the exhaust system. Drip-drip it goes, puzzling interested seven-year-olds.
In so-called “chemically correct” fuel-air combustion, the proportions of fuel to air are such as to produce only water and carbon dioxide—no left-over hydrogen, carbon, or oxygen. This is how the oxygen sensors of modern digital fuel injection systems operate by monitoring leftover oxygen).
If there is too much fuel for the oxygen supplied, the mixture of fuel and air is said to be rich. If there is too much air for the fuel supplied, the mixture is termed lean. In either case, because the uncombined excess molecules carry away some heat but generate none, energy release is reduced and flame speed slows.
Experiment has shown that an electric spark cannot reliably ignite fuel-air mixtures that are more than either 20 percent lean (approximately 18 parts air to one part fuel) or 20 percent rich (10 parts air to one fuel). Although there are now engines that use other means of ignition to burn mixtures leaner than 18-to-one, let’s stick to spark ignition for now.
Carburetion
The earliest way to create an ignitable fuel-air mixture was by evaporation. The engine’s intake air was passed through layers of fuel-soaked cloth on its way to the cylinder. This was the wick carburetor. Because of the difficulty of controlling the mixture, it worked best at constant engine load.
In 1893 Wilhelm Maybach patented the spray carburetor. It made use of the fact that moving air has less pressure than still air: The partial vacuum in fast-flowing intake air allowed the higher pressure in a nearby fuel reservoir (aka float bowl) to push fuel into that intake airstream.
As air moves through the carburetor, fuel is pushed into the airstream. (Mark Hoyer/)
In the collision of fuel with intake air moving at hundreds of feet per second, the fuel stream was broken up into particles held in roughly spherical form by the fuel’s surface tension (surface tension is what allows the insects known as “water striders” to walk on water). The pressure of the air upon hitting such droplets first flattens, then punches in their upstream faces. Surface tension then pulls them into the form of tiny rings whose wobbly instability causes them to break up into even smaller droplets. The higher the air velocity, the finer the droplet size.
Droplet breakup increases the total fuel surface area exposed to the surrounding air, accelerating fuel evaporation.
Fuel Injection
In the port fuel injection employed on most motorcycle engines of today, the fuel injector is a solenoid-operated valve, usually with eight or 12 tiny holes through which fuel sprays. The droplet breakup process described above operates here as well, but is given a head start by the finer initial droplets produced by the multi-hole injector.
Port fuel injection produces finer fuel droplets than a carburetor. (Yamaha/)
Early single-orifice injectors tended to work less well than carburetors because many of the droplets they produced were quite large. At least one brand of early fuel-injected bike engine puzzled owners because engine oil level increased during use! Large unevaporated fuel droplets were able to reach the cylinder walls and be scraped down into the crankcase by the piston’s oil scraper ring, diluting the crankcase oil and gradually raising its level.
In both carburetors and port fuel injection, fuel moves toward the engine in three forms: as evaporated fuel vapor, as some remaining droplets, and as “wall wash” (liquid fuel sliding along the inner walls of the intake duct).
Direct Injection
A third technology of mixture formation is Gasoline Direct Injection (GDI), by which an injector operating at much higher fuel pressure (typically up to 2,200 psi) sprays fuel directly into the cylinder’s combustion chamber.
Time
Time is an important variable in mixture formation. Because carburetors are located at some distance from the intake valve(s), they provide the longest “time-of-flight” during which fuel droplets and wall wash can evaporate. Port fuel injection of the kind presently most used on auto and motorcycle engines provides shorter time-of-flight, so for engines of high peak rpm a second set of “showerhead” injectors may be provided, located just above each intake pipe’s bellmouth. At higher revs, showerhead injectors take over most of the fueling, providing more time for droplet evaporation.
Fuel evaporation can also be sped up by heating the fuel. Many snowmobiles were given carburetor fuel bowls heated by engine coolant. In F1 racing, Honda in the 1980s era of turbo engines running on highly antiknock but slow-evaporating toluene-based fuel were provided with fuel heaters. Carbureted auto engines for many years assisted fuel evaporation by use of exhaust-heated intake manifolds.
In general, the harder a blender of racing fuel tries to increase its octane number (a measure of resistance to engine knock or ping), the slower-evaporating the fuel becomes.
Another technique of assisting fuel evaporation was to point the injectors upstream. Not only did this increase the relative velocity of collision with the airflow, it also extended evaporation time.
The shortest time of evaporation is provided by GDI, injecting into the combustion chamber itself. To compensate, injection pressure is increased. Higher injection pressure accomplishes two things:
It can get the desired fuel charge into the cylinder quickly enough to leave adequate time for evaporation.It increases the velocity with which the fuel hits the air, thereby achieving smaller fuel droplet size.
Direct injection is not a new technology, having been applied to German military aircraft piston engines in World War II, and in the mid-1950s to Mercedes racing cars. It was later (late 1980s to early ‘90s) developed in a form suitable for two-stroke engines as a means of greatly reducing their UHC exhaust emissions.
It appears that once fuel droplets become 10 microns (0.00036 inch) or smaller they behave during combustion as if fully evaporated.
Modern auto and motorcycle port fuel injection is not constant-flow. Each cylinder has its own solenoid-operated injection valve, located under the throttle butterfly. The valve is opened by sending current to the solenoid, and is closed by a spring when the current is switched off. Fuel delivery is controlled by how long the injector is held open each 720-degree cycle, called “on-time.” The accurate control over air-fuel mixture this makes possible is an essential technology in delivering high performance while meeting exhaust emissions limits. Older riders and drivers will remember the “stutter-and-stall” era of the 1970s and ‘80s, when lean carburetor jetting was required to meet those limits.
There have been many constant-flow injection systems, most famous of which is Hilborn injection, used on Champ cars from the late 1940s to the 1970s, and remaining in use in specialized off-road applications to this day.
