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injection v carbs
- Thread starter qunvat
- Start date
Allow me to help shed some light on this sir.
Carburetors are basically a low pressure fuel metering system that uses the air drawn in on the induction stroke to create an accelerated air flow across a jet or jets to create and air fuel "mist". On an SU carb the incoming air lifts a piston which in turn raises a needle in a seat allowing more fuel under light pressure to be exposed to the in rushing, the more you put your foot down the more air get sucked in and the more fuel/air mixture gets delivered. On a weber the in-rushing air (once again controlled by a butterfly) moves around a fixed venturi and produces similar results.
Great so far, all I have to do is get the jets right and all will be well right -- wrong great in theory but in the real world its a little more tricky than that. On an SU for instance you only have one needle which is tapered -- getting that right at all engine speeds and loads is nigh on impossible -- enter the weber where you have a system of jets, air tubes and drillings. This system gives you a greater degree of tunability over the SU but still is very tricky and expensive to get even close to right. All of the above only deal with carburation and in no way touch the ignition system yet the two are so interdependent that there needs to be a better way -- there is its called fuel injection.
Fuel injection comes in many forms but all of them work in a similar manner (but deliver different results)
High pressure fuel (mine is at 120 psi) is pumped constantly to a fuel rail and governed down to a sensible constant pressure (3.5 bar or about 52 psi) where it feeds one or more (in my case 6 - one for each cylinder) solenoid valves called injectors. these injectors are controlled by an onboard computer which knows precisely the position of the piston in each cylinder -- at exactly the right moment these injectors open for a split second (variable) and delivers the exact amount of fuel in a controlled mist. A number of sensors around the engine (temperature, pressure, crank position etc) keep the computer informed as to the engine speed, load and the temperature of the coolant and incoming air and in turn the computer varies the opening times of the injectors.
The advantage of such a system is precise metering of the fuel mixture at ANY speed, load and temperature.
Such a system is great for economy and power as the atomization of the fuel and precise metering is far better than you can ever get on a carb
One stage above this is to let the onboard computer control the ignition as well so doing away with the distributor
Distributors are ok but are mechanical and thus wear out over time giving "scatter" this is when the spark is triggered inaccurately and this is igniting the fuel at the wrong time.
Even a new distributor is hampered by its inability to be variably tuned
If you let the computer control everything then the engine is under complete control -- fuel is metered precisely at exactly the right time, the mixture is ignited at exactly the right time.
A system such as this can produce greater HP than a carburated system as now you have the two (fuel and spark) working together.
So advantages of FI are
Fuel economy, power engine longevity, drivability
Beware though don’t think that by bolting on the Nissan plenum type system off the 280ZX that you will have a tarmac eating fire breathing monster. The Nissan system was designed primarily with economy and emissions in mind and not power.
Cons are
Cost in engineering terms, complexity, more stuff to go wrong -- lalalalala
Carburetors are basically a low pressure fuel metering system that uses the air drawn in on the induction stroke to create an accelerated air flow across a jet or jets to create and air fuel "mist". On an SU carb the incoming air lifts a piston which in turn raises a needle in a seat allowing more fuel under light pressure to be exposed to the in rushing, the more you put your foot down the more air get sucked in and the more fuel/air mixture gets delivered. On a weber the in-rushing air (once again controlled by a butterfly) moves around a fixed venturi and produces similar results.
Great so far, all I have to do is get the jets right and all will be well right -- wrong great in theory but in the real world its a little more tricky than that. On an SU for instance you only have one needle which is tapered -- getting that right at all engine speeds and loads is nigh on impossible -- enter the weber where you have a system of jets, air tubes and drillings. This system gives you a greater degree of tunability over the SU but still is very tricky and expensive to get even close to right. All of the above only deal with carburation and in no way touch the ignition system yet the two are so interdependent that there needs to be a better way -- there is its called fuel injection.
Fuel injection comes in many forms but all of them work in a similar manner (but deliver different results)
High pressure fuel (mine is at 120 psi) is pumped constantly to a fuel rail and governed down to a sensible constant pressure (3.5 bar or about 52 psi) where it feeds one or more (in my case 6 - one for each cylinder) solenoid valves called injectors. these injectors are controlled by an onboard computer which knows precisely the position of the piston in each cylinder -- at exactly the right moment these injectors open for a split second (variable) and delivers the exact amount of fuel in a controlled mist. A number of sensors around the engine (temperature, pressure, crank position etc) keep the computer informed as to the engine speed, load and the temperature of the coolant and incoming air and in turn the computer varies the opening times of the injectors.
The advantage of such a system is precise metering of the fuel mixture at ANY speed, load and temperature.
Such a system is great for economy and power as the atomization of the fuel and precise metering is far better than you can ever get on a carb
One stage above this is to let the onboard computer control the ignition as well so doing away with the distributor
Distributors are ok but are mechanical and thus wear out over time giving "scatter" this is when the spark is triggered inaccurately and this is igniting the fuel at the wrong time.
Even a new distributor is hampered by its inability to be variably tuned
If you let the computer control everything then the engine is under complete control -- fuel is metered precisely at exactly the right time, the mixture is ignited at exactly the right time.
A system such as this can produce greater HP than a carburated system as now you have the two (fuel and spark) working together.
So advantages of FI are
Fuel economy, power engine longevity, drivability
Beware though don’t think that by bolting on the Nissan plenum type system off the 280ZX that you will have a tarmac eating fire breathing monster. The Nissan system was designed primarily with economy and emissions in mind and not power.
Cons are
Cost in engineering terms, complexity, more stuff to go wrong -- lalalalala
zedhead260
Well-Known Forum User
They are essentially the same bit of kit.qunvat said:
toopy
Club Member
Quote from march edition of retro cars:
In truth, fuel injection has been around for as long as the carburettor, and it was largely because
of the practical difficulties of making early carburettors reliable that the idea of squirting the fuel
under pressure into the engine was concieved.
The first fuel injection systems appeared in 1905, while the birth of the aircraft industry gave a further boost to development
when it was discovered that carburettors were prone to icing abd catching fire in the air: the wright brothers equipped all their early fliers with their own home-made injection system.
World War 2 saw the technology productionised and it started appearing in road cars in the 50's.
Early systems were mechanical, like the 'Kugelfischer' system found in the BMW 2002 tii. Similar to a diesel injection system.
My fingers hurt
Anthony
In truth, fuel injection has been around for as long as the carburettor, and it was largely because
of the practical difficulties of making early carburettors reliable that the idea of squirting the fuel
under pressure into the engine was concieved.
The first fuel injection systems appeared in 1905, while the birth of the aircraft industry gave a further boost to development
when it was discovered that carburettors were prone to icing abd catching fire in the air: the wright brothers equipped all their early fliers with their own home-made injection system.
World War 2 saw the technology productionised and it started appearing in road cars in the 50's.
Early systems were mechanical, like the 'Kugelfischer' system found in the BMW 2002 tii. Similar to a diesel injection system.
My fingers hurt
Anthony
grolls
Well-Known Forum User
well ya both smart asses or you been searching on google!
yes MR Diesel did design it between 1892-1897...some dispute there a little like the 240z!
MR Bosch followed this up in 1939 with an aviation version that was fitted to a BF109 Messerschmitt.. a tenner heading the clubs way....next time i will make it not so easy!
yes MR Diesel did design it between 1892-1897...some dispute there a little like the 240z!
MR Bosch followed this up in 1939 with an aviation version that was fitted to a BF109 Messerschmitt.. a tenner heading the clubs way....next time i will make it not so easy!