Why do petrol driven cars require more petrol when cold?

[qube]

Never had a satisfactory explanation as to why cars are so much more inefficient when they're cold but Diesels are the same no matter what temp.

Anyone on here who can clear it up?

Cheers

Matthew

[MXM]

Fule burns better when engine is warm. Lubrication works much better when warm. Whole transmission works smoother when worm... So you need more energy to do the same job when cold -> mixture get's richer, emissions grow insane and efficiency drops.
And I'm pretty sure it's the same with diesel. Actually diesel fuel gets "thicker" when cold, so it's pain in the ass to sart it when it's something like -25C or lower.

[mmap]

+ plus you need a different type of diesel when cold... (or you can mix some petrol into it yourself if you like DYI)

[bone]

^you mean winterdiesel, caus ordinary diesel would freeze. qube is talking about driving with a cold engine

before engines had injection there was the choke, which cut off a part of the airsupply, resulting in less air, and in a vaccuum, whuch sucks more fuel through the jets

[qube]

MXM if it was just that then the mixture on a diesel would bich richer when the car is cold too it doesnt.

[bone]

a diesel engine works with sponatious combustion, that's why you have to wait a few seconds before starting, the engine is pre-heated

[qube]

Found this:

When the air is cold, it is denser, and hence has more oxygen per unit of volume or capacity. You therefore need more petrol to approximate the ideal ratio of air and fuel,hence you choke the car by cutting off some of the air flow through the carburetor. You are not actually adding more fuel, but instead cutting back on the air flow.

However, if you did that, there would not enough air/fuel flow to let the engine run, so, most carb cars have a fast idle cam which sets the idle at a higher rpm than normal. It is this faster idle, which also tends to make the carb come off its idle jest, and move onto its main jets that tend to cause a carbed car to use more fuel at cold start, and especially at idle.

The best cold start process is to start and drive away immediately and not let the car idle on choke. That way, you do not tend to have poor/heavy fuel consumption on cold start.

FI cars work the same way except the the sensors tell the ECM/PCM that the engine coolant is cold and the air temp is cold so they ramp up the injector pulse width to squirt in more fuel. They also tend to close the IAC to shut of some air and raise the cold idle speed too, but it is all done far more efficiently than the old carb cars.

[bone]

an individual cam just for idling

never heard of that before

can you give a link pls?

[krummi]

Originally Posted by qube

The best cold start process is to start and drive away immediately and not let the car idle on choke. That way, you do not tend to have poor/heavy fuel consumption on cold start.

The problem in this is that when you drive the wind keeps the engine cooler than if you let the car stand and idle for a while. Of course if you are just doing a couple mile shopping trip at low speed, this isnt a issue. Here some people cover the front air intakes of their car with cardboard or something during the winter so the engine would heat up faster.

Of course best is to have a engine-block heater.

[Leppy]

When the air is cold, it is denser, and hence has more oxygen per unit of volume or capacity. You therefore need more petrol to approximate the ideal ratio of air and fuel,hence you choke the car by cutting off some of the air flow through the carburetor. You are not actually adding more fuel, but instead cutting back on the air flow.

Ummm I this I believe is wrong because you only use the choke on start up. By what your suggesting, the car should run the whole time with the choke on which engines don't do.

The best cold start process is to start and drive away immediately and not let the car idle on choke. That way, you do not tend to have poor/heavy fuel consumption on cold start.

This is also makes no sense at all. Your car uses a choke until it is up to its operating temperature regardless if it is moving or not. Also what you are suggesting is to put the engine under load straight away without letting the car to warm the oil to lubricate the engine. This in my book is a very bad practice indeed.

[chaos386]

I know in our Sienna the idle speed is higher and the shift points are later when the engine is still cold. It's supposed to warm the engine and catalytic converters up as quickly as possible to improve emissions.

[adrianpike]

Originally Posted by Leppy

Ummm I this I believe is wrong because you only use the choke on start up. By what your suggesting, the car should run the whole time with the choke on which engines don't do.

It is true. A choke reduces the airflow into the carburettor. This has the effect of richening the fuel mixture, as the manifold vacuum remains the same.

Originally Posted by Leppy

The best cold start process is to start and drive away immediately and not let the car idle on choke. That way, you do not tend to have poor/heavy fuel consumption on cold start.

Incorrect, but only partially.
The reason a car requires more fuel during startup has to do with multiple things. Mainly, gasoline burns most efficiently at a fairly warmish temperature (can't remember off the top of my head, I could find my notes), and anything less will be inefficient. This is the reason for thermostats in an IC engine, keeping the engine as close to the point of maximum efficiency is best.
Under cold start, the engine will be less efficient, which means that more fuel will have to be injected/squirted/sprayed for the engine to run smoothly at stoichiometric. In the case of my car, I've got my warmup enrichments set to 160% at the coldest (-20 C), which is a touch rich. I don't mind having a touch rich, as I don't have a cat to worry about destroying, and because burning a little more fuel is worth it for me to have quick, reliable starts.

[Leppy]

Originally Posted by adrianpike

It is true. A choke reduces the airflow into the carburettor. This has the effect of richening the fuel mixture, as the manifold vacuum remains the same.

Incorrect, but only partially.
The reason a car requires more fuel during startup has to do with multiple things. Mainly, gasoline burns most efficiently at a fairly warmish temperature (can't remember off the top of my head, I could find my notes), and anything less will be inefficient. This is the reason for thermostats in an IC engine, keeping the engine as close to the point of maximum efficiency is best.
Under cold start, the engine will be less efficient, which means that more fuel will have to be injected/squirted/sprayed for the engine to run smoothly at stoichiometric. In the case of my car, I've got my warmup enrichments set to 160% at the coldest (-20 C), which is a touch rich. I don't mind having a touch rich, as I don't have a cat to worry about destroying, and because burning a little more fuel is worth it for me to have quick, reliable starts.

Learn how to quote mate. I didn't say that second part as you can see by my rebuttal directly underneath it.

(in response to the part bolded) If you car is running rich then its not running at stoichiometric.

[adrianpike]

Originally Posted by Leppy

Learn how to quote mate. I didn't say that second part as you can see by my rebuttal directly underneath it.

(in response to the part bolded) If you car is running rich then its not running at stoichiometric.

Sorry about the quote screwup, didn't pay enough attention, I guess. My apologies for misquoting, I'll go edit my post now.
And I'm not totally sure where in the bold part that you read that my car is running rich, I mentioned that later, and I explained that it's intentional to assist in cold-start reliability.

[Leppy]

The part about the more fuel being injected when cold started. With the choke on, there is less air more fuel ie. not stoichiometric. Thats the point I was making.

[Viper007Bond]

Originally Posted by qube

When the air is cold, it is denser, and hence has more oxygen per unit of volume or capacity. You therefore need more petrol to approximate the ideal ratio of air and fuel,hence you choke the car by cutting off some of the air flow through the carburetor. You are not actually adding more fuel, but instead cutting back on the air flow.

Ah, so that's why I gotta pull the choke on my Triumph to get it to start up when it's cold out.

[Leppy]

^^^ Viper I'm pretty sure thats wrong.

Cause if what qube said is true then people who drive in cold climates, regardless of whether the engine is hot or not, would have to run with the choke on all the time and that doesn't happen.

[Greatgraddage]

You need the choke just to start her up, once the engine is going it'll be fine. My only experiance with chokes is on marine outboard 2 strokes, we use the choke on some engines because they are tempremental when starting up, no matter what the conditions they won't start without some choke, but as soon as they are running the choke goes off, and if they have been running for some time they will usualy restart without choke if they stall or are turned off for a short time.