How can aerosoled fuel injected directly into the cylinder burn better than evaporated gasoline mixed in air vacuumed into the cylinder.
Thank you.
A carb puts out big droplets
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I thoughthathe droplets evaporated before the fuel/air entered the cylinder.
The vacuum would further help the gasoline outgas.
Thank you.
A carb doesn’t vaporize fuel. It is sucked into the airstream by the “venturi effect”. The droplets are larger than fuel injected at 50 psi would be.
Additionally, fuel injection can be timed to enhance the velocity of air entering the cylinder by “shooting” a mass of fuel at the exact right time. Like pushing someone on a swing. Timing is important.
A carb works like this… (very cool video)
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Oh, like when we use the fire hose fan spray < to blow smoke out of a window.
Like slight supercharging!
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It’s sprayed directly into the chamber versus a wet intake approach which skews the fuel getting to the chamber. As noted, neither are perfect and both initially produce droplets. You are aware that the engine gets hot? What would be better, atomized fuel contacting the intake manifold or going directly into a hot combustion chamber to vaporize almost instantly?
All of this stuff has been studied and tested exhaustively for the best balance of power production, emissions and fuel efficiency. To the point that the crevice volume at the top ring to piston top is considered…
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A better question is why use direct injection instead of port injection. They’re now nearing the limits on particulates for gasoline engines with direct injection, not nearly the same problem with port injection, and port injection reduces inlet deposits compared to DI.
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Good question. I don’t think fuel injected gasoline burns any better or any cleaner (more thoroughly) than it would in a perfect carb. But most carb designs are far from perfect, and definitely less perfect than a corresponding computerized fuel injection system, especially at obtaining the correct air/fuel mixture over a wide range of driving conditions. It’s probably possible to find a vdo of a fuel injector spray spraying gasoline. I used to do that test on my VW Rabbit routinely, similar looking to how one of those round water sprinkler sprays water, quite visible-sized droplets. Carbs must not be that different b/c smaller carb’d engines (like the Toyota 4A) got pretty similar mpg to their fuel injected counterparts later (Toyota 4AFE).
But they couldn’t meet emissions regs. That’s a huge benefit of EFI - precise control of the amount of gas injected to each cylinder. And high pressure injectors can create a finer mist than a carb can, improving evaporation and combustion.
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Direct injection into the combustion chamber can improve power and economy even more than injection into the intake air stream.
It can allow higher compression ratios because it cools the compression charge. The extremely high pressure further atomizes fuel and you can double or triple hit the injector at low speeds to optimize the combustion.
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Forgetting droplet size and pretty much everything else, the simple explaination is that a Carb going downhill, with your foot off the gas, will continue to inject gas into the engine while a modern Fuel Injection won’t.
i.e. As long as your engine is injecting air, a carb will mix gasoline with it, whether it’s wanted or not.
Further, carbs rely on a very primitive mechanical means of mixing gas with air by venturi effect, with the result that a 1.2 liter, 4 cylinder sports car engine may get only 50 HP and 10 MPG.
Cars like the 133 HP, 30 MPG, 1.3 liter, 3 cylinder turbo Buick would not be possible without fuel injection and modern technology
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Toyota 3AC, 1.5l, 62 HP in an 86 Tercel, 23/29 mpg, epa
Toyota 4AFE 1.5l, 93 HP in a 92 Tercel, 27/33 mpg, epa
Toyota 4AC, 1.6l, 75 HP in 82 Corolla, about 22 mpg, fuelly.com
FI is more precise than carburetors. The fuel droplet size is smaller and more consistent in FI vs carb. The smaller and more consistent droplet size allows the fuel to burn quicker meaning that more fuel burns in the cylinder vs in the exhaust pipe.
My 61 Fiat Cabrolet (later spider) with 1.2L 4 cylinder and Weber 28/32 Carb rated at 59 CV/65 SAE hp and got 25 mpg around town and 50 mpg on the I-40 Calif to Tenn road trip.
So a given fuel injected engine will run with NO fuel as long as your foot is not on the gas pedal?? interesting…
One of the posters here doesn’t seem to believe this is possible … lol … but the fuel injection seems to be completely shut off in that situation on my manual trans Corolla. I can feel it happen as I take my foot off the pedal at 35 mph to approach a red stop light, slowing in 3rd gear. When the car reaches about 10 mph I can then feel the effect of the fuel injection turning back on, done by the computer to prevent the engine from stalling. The computer doesn’t know which gear is selected, but knows the throttle is in the idle position, and it knows the engine rpm and the vehicle speed, so it must figure it out from that. I’m not sure if it turns off the fuel injection on automatic trans versions of the same car though, maybe not.
Maybe yes! Auto trans cars do that too.
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Some engines use a fuel-cut strategy during deceleration at engine speeds above 1500 rpm, but not all vehicles do this.
Pay no attention to this post… 
I deleted it but that can’t be done here…
I got it (I think)… the engine is still turning basically the same rpm as long as it is in a gear due to the mechanical linkage to the transmission be it manual or auto and to the drive wheels… I guess the secondary ignition is dry firing the whole time?? and as soon as the injector adds fuel to the engine it fires right up like a roll start at a higher rpm then pushing it across the parking lot…
I guess as long as the engine is still rotating fast enough that the converter will spin the pump fast enough for the computer to redirect the flow to engage the lock up (TTC) to make it a mechanical link and still maintain sufficient pressure/flow to the rest of the transmission…
It can’t be anywhere near that bad.
I think it does. The Camry does.
Fuel injection can make the mixture more lean at light load and more rich under heavy load. It can adjust the mixture according to oxygen sensor inputs. This is something a carburetor can’t do without complicated electro mechanical controls.
Direct injection can inject the gasoline later to prevent pre ignition. This allows the compression ratio to be higher.
It’s called Deceleration Fuel Cut Off (DFCO).
My 1985 fuel injected Accord had it.
The RPM would jump up a little ~1500 rpm as the car slowed down and the fuel came back on.
There’s engine braking anyway when coasting down in gear, throttle closed.
Just a little more braking with the fuel off.
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