Hello, I have a question that bothers me for a while (since last cold, cold winter in Poland).

It is about cold starting and 2 different strategies of getting the engine to running temperature (warm-up to 90 deg. C).

One strategy is used for carburettor cars (like mine) and involves a so called 'rapid idle' meaning the engine will run rich and fast (up to 2300 rpm!!) then gradually reducing idle speed to 800 rpm when warmed up.

The other strategy is used on injection cars that run (to my knowledge) rich in open loop, but at a considerable lower speed (some 1200 rpm).

My concern is about noise and consumption during warming up.

First strategy revs the engine too much IMO and probably eats more fuel than cars using the second strategy. I might set up rapid idle to a lower speed, but warming up would take longer.

Why ECU cars are using lower rapid idle? Do they have other means for a quicker warm up?

What am I missing? Sorry if my English makes me a bit confused.

If I'm understanding you correctly, we'd call the elevated idle revs you get on a cold start "fast idle" rather than "rapid idle".

I presume that you know how a carburettor basically works (venturi, jet and needle when applicable).

Choking a carb closes a butterfly in the choke (primary choke(s) for compound multi-barrel carbs), and lowers the jet, allowing more fuel to be drawn for a given amount of air flow. Obviously, this causes the engine to run rich increasing the idle speed and fuel consumption. This is a fairly crude process, because it is controlled by either a cable (like a throttle, but with typically 1 or 2 inches of travel to 90 degrees of butterfly rotation) or a hot wax cyliner like a thermostat (on carbs I've looked at except the SU HIF44 which is irrelevant here because Skoda never used it or anything similar).

ECU cars also need more fuel for a cold start, but have much more delicate control over the enrichment process (how delicate depends on the ECU, but an 8-bit word would allow 256 discrete positions), allowing them to use a leaner mixture at given engine temperature.

My bad, is fast idle indeed.

I understand your point of view, but there are some corrections: Felicia choke is not cable operated, but semi-automatic (regulated by vacuum and coolant temperature. The choke valve is opening in a linear way (better than 256 steps) and throttle valve is also closing quasi-linear.

What I can't figure out is why carburettor needs howling the engine at 2300 rpm?? compared to 1200 rpm for MPI cars.

I don't think it is anything to do with the technologies. It is just that the later ECU controlled cars were build at a time when emissions/economy are more important than quick warm up. ECU controlled engines were introduced at the same time as catalytic converters and running very rich with a catalytic converter will damage it.

Let me put it this way: how long does it take the 1.3 MPI engine to warm up from 20 to 90 deg. C ?

I will replicate the process as close as possible on my 1.3 carburettor car.

Carburettors do not atomise fuel as well as injection systems do. When the engine is cold this is a bigger issue - cold engine/manifold as well as fuel/air. Therefore an injection engine can run a much lower speed than a carb engine as the carb engine will rely on higher engine speed (and therefore air speed into the engine) to get sufficient atomisation and smooth enough running. An injection engine obviously still suffers the same issue in terms of atomisation, but the MPi far less so as the fuel is injected so close to the inlet valve - indeed just about straight onto it (hence modern engines moving to that design); the old SPi ones put the fuel in where the carb is, so suffers more so, but less than a carb engine.

Thank you, djaychela. Now all makes sense.

One last question, please. From your knowledge, is there any device capable of improving atomization pre or post carburettor?

For now I am thinking of some sort of ultrasonic probe? capable of inducing cavitation in fuel droplets and make them implode combined with a vortex air intake and a 'hedgehog' preheater.

No such device exists as far as I know, but I've never looked. It doesn't sound like something that is simple to achieve because of the physics of the situation, let alone then achieving it in the context of a moving car's engine bay.

Plenty of "snake oil" devices that claim to improve atomisation have been offered for sale. None have stood up to the inspection of an independant 1000 mile test by a reputable trade body.

If you can find a set of magnets from a scrap computer hard disc or a cathode ray tube for free, you could try the magnets idea, but I'm less than optomistic.

If you can find a set of magnets from a scrap computer hard disc or a cathode ray tube for free, you could try the magnets idea, but I'm less than optomistic.

Getting the magnets part is easy, but I don't see any science behind 'magnetic atomization". Do you?

"snake oil" device? Please be more specific.

Getting the magnets part is easy, but I don't see any science behind 'magnetic atomization". Do you?

No, particularly since fuel is non-magnetic! I just figure that, if you can get some powerful magnets for free it could be worth trying.

"snake oil" device? Please be more specific.

"Snake oil" in this context is a substance or device which has near miraculous things claimed of it, but which is ineffective or sometimes has adverse effects. http://en.wikipedia.org/wiki/Snake_oil It's such a common phrase in both popular culture and Westerns that I assumed you'd have come across it before.

"Snake oil" ... is such a common phrase in both popular culture and Westerns that I assumed you'd have come across it before.

Sorry, never heard of because Poland is a Slavic country. But I don't mind learning new expressions.

What is really hard to read/understand is the entire forum is uma's replies. Shouldn't be mandatory using first capital letters, commas, dots and all other English punctuation rules?

I will struggle this fall and winter to build and test few atomization devices because this is the best low consumption solution. Did you know that 1 gallon of liquid gasoline is 160 gallons of gasoline vapors? See http://presscore.ca/2011/?p=6898

That blew my mind...

Hi, what's the normal fuel consumption for a Felicia 1.3L with carburettor?

Thanks!

When the fuel gauge is at its mínimum i.e. the warning panel orange light bulb is on, how many kilometers or liters do I have to get to the nearest gas station?

6 liters.

Thanks. Now What's wrong when the car is descending a long slope and the engine stops working?

Too many causes. I remind you (again) this topic is called Rapid Idle vs ECU and I already answered to 2 off-topic questions.

Create a new topic, give as many relevant information as you can, and we will try to solve your problem.