I presume that most of us have had used Choke to start up their motorcycle engines, once in their lifetime. It is being mostly used during winters to avoid starting problems. Ever wondered how it works and what are the technical aspects behind it. How does it help an engine to ignite so quickly without any problem? Is it something related to the fuel intake of the engine, as it is said that the usage of choke greatly increases the fuel consumption and hence it is never recommended for continuous usage.
The crux of it is - The engine needs its fuel as a VAPORIZED mixture. The conversion of the fuel (petrol/diesel), from liquid form in the fuel tank, to the vaporized form, is done inside the carburettor of the motorcycle. It is inside the carburettor, that the fuel is mixed with a jet of clean air, coming from the air filter. This mixture of fuel and clean air is then sent inside the engine for ignition. Especially during winters or when a vehicle has been idle for many days, the engine might face starting problems and needs a little warm up. This can be achieved by providing such a mixture to the engine which is rich in fuel and has less of air content in it. This is where choke comes into the picture. The application of choke reduces the air intake and increases the fuel content in the mixture, when applied. This extra amount of fuel will make the mixture rich. This means more fuel content in the fuel-air mixture and thus quick ignition inside the engine.
In case of the diesel engines, they have a heater which first vaporizes the mixture and then the compression ignites it.
Ignition problems are faced mostly in winters because the fuel is cold and thus it takes more time to vaporize inside the carburettor and then ignite. It is for the same reason that after starting, the vehicle "coughs", and takes a while for the engine to return to its normal rhythm. It’s one of the reasons that it is recommended to let the engine warm up before starting to move. The other reason being a whole different discussion for some other day :)
When the temperatures are low or it has been a while since the engine was last started, the fuel in the mixture condenses or joins up to form droplets which are difficult to burn. When we pull the choke it has a dual effect - one it stops or restricts the air supply from the air filter (that’s why the word “choke”) and two, it creates a low pressure or partial vaccum which exerts a pull effect on the fuel inlet thereby pulling in more fuel. The idea is to have more fuel in the combustion chamber with the hope that enough of it is in the atomized or aerosol form.
The restricted air supply makes the mixture richer and thus it burns with more ease & that too viciously though very inefficiently, with not enough air for complete combustion. The high energy generated also helps fight off the obstruction to the free movement of the crank presented by the cold engine oil which tends to become thick, heavy and sluggish.
Besides these reasons, running with the choke on may get the engine flooded with fuel and disturb the air-fuel mixture to the extent that it prohibits combustion (problem of “over” in vehicles, specially Enfields). If the vehicle does not start in 4-5 attempts, it’s advisable to turn off the choke and try to force-start or push-start it.
The application of choke also helps when we are out of fuel. What happens in this case is that the remnants of fuel in the pipe, carburettor etc., all get pulled in because of the greater force exerted by the partial vacuum and we get the precious few hundred meters.
Another point to note is that there are fuel additives & ethanol blended fuels available that prevent fuel from condensing or forming heavy droplets. Not sure about their availability in India, but they are there in cold countries.
As far as the technicalities of the choke design itself are concerned, there are many variants, from simple ones that just close the air inlet, to complex ones with thermostat controlled inlet openings.
My two cents or a few. Cheers!! Ride Free!!
The crux of it is - The engine needs its fuel as a VAPORIZED mixture. The conversion of the fuel (petrol/diesel), from liquid form in the fuel tank, to the vaporized form, is done inside the carburettor of the motorcycle. It is inside the carburettor, that the fuel is mixed with a jet of clean air, coming from the air filter. This mixture of fuel and clean air is then sent inside the engine for ignition. Especially during winters or when a vehicle has been idle for many days, the engine might face starting problems and needs a little warm up. This can be achieved by providing such a mixture to the engine which is rich in fuel and has less of air content in it. This is where choke comes into the picture. The application of choke reduces the air intake and increases the fuel content in the mixture, when applied. This extra amount of fuel will make the mixture rich. This means more fuel content in the fuel-air mixture and thus quick ignition inside the engine.
In case of the diesel engines, they have a heater which first vaporizes the mixture and then the compression ignites it.
Ignition problems are faced mostly in winters because the fuel is cold and thus it takes more time to vaporize inside the carburettor and then ignite. It is for the same reason that after starting, the vehicle "coughs", and takes a while for the engine to return to its normal rhythm. It’s one of the reasons that it is recommended to let the engine warm up before starting to move. The other reason being a whole different discussion for some other day :)
When the temperatures are low or it has been a while since the engine was last started, the fuel in the mixture condenses or joins up to form droplets which are difficult to burn. When we pull the choke it has a dual effect - one it stops or restricts the air supply from the air filter (that’s why the word “choke”) and two, it creates a low pressure or partial vaccum which exerts a pull effect on the fuel inlet thereby pulling in more fuel. The idea is to have more fuel in the combustion chamber with the hope that enough of it is in the atomized or aerosol form.
The restricted air supply makes the mixture richer and thus it burns with more ease & that too viciously though very inefficiently, with not enough air for complete combustion. The high energy generated also helps fight off the obstruction to the free movement of the crank presented by the cold engine oil which tends to become thick, heavy and sluggish.
Besides these reasons, running with the choke on may get the engine flooded with fuel and disturb the air-fuel mixture to the extent that it prohibits combustion (problem of “over” in vehicles, specially Enfields). If the vehicle does not start in 4-5 attempts, it’s advisable to turn off the choke and try to force-start or push-start it.
The application of choke also helps when we are out of fuel. What happens in this case is that the remnants of fuel in the pipe, carburettor etc., all get pulled in because of the greater force exerted by the partial vacuum and we get the precious few hundred meters.
Another point to note is that there are fuel additives & ethanol blended fuels available that prevent fuel from condensing or forming heavy droplets. Not sure about their availability in India, but they are there in cold countries.
As far as the technicalities of the choke design itself are concerned, there are many variants, from simple ones that just close the air inlet, to complex ones with thermostat controlled inlet openings.
My two cents or a few. Cheers!! Ride Free!!
