Coolant and Inlet Manifolds 2

[scrimshaw]

I thought this was a simple enough question when I first asked it but turns out I am still not sure of the answer.
Why do some carbed manifolds have coolant running through them and some don't?
Even on the same engine there can be different designs where sometimes coolant flows and sometimes not.
Anyone explain it in simple turns for a guy who knows little about engine science?

 

[BillClark]

do you mean where the (hot) coolant runs under or around intake runners. If so I beleive it has to do with the heated surface aiding in vaporization of the fuel mainly where it tends to puddle (stick) to the port floor and walls which will help combustion efficiency (reduce harmful emissions). A gasious mixture is easier to ignite/burn compleatly than large fuel droplets. Carbed engines are notorious for poor fuel atomization at low speeds anyway.
May be multiple reasons why this effect is not desirable under all conditions. Mainly when the decreased air density from the heated air/fuel would hurt power output.
Plumbing hot exhaust gasses under the intake was common years ago to create the same effect

 

[BrianPetersen]

In some cases that I'm aware of, the versions sold in countries where a lot of cold/wet operation was expected had the coolant passages (to resist carb icing), and the versions sold in warmer and/or drier areas did not (to save money and because heating the intake charge costs a little bit of power).

 

[SomptingGuy]

And then of course there were some well known Italian V-twins that suffered horrendously from carb icing, but never had a de-icing kit fitted as standard.

- Steve

 

[patprimmer]

Intake manifolds can be heated in several ways.

1) Engine water being circulated through jackets where ever the designer decides it needs heat.

2) Exhaust gas being passed through jackets in a similar manner to the water.

3) The manifiold being placed so that engine oil splashes on the bottom of the manifold such as over the valley in V type engines.

4) Air being preheated by passing it over an exhaust pipe before it enters the manifold.

All these methods are for several reasons.

1) To aid in evaporation of the fuel, especially in cold starts and low speed. This is most critical on an engine with a central carby over a low mounted plenum with runners running up and off in various directions and is less critical on multi carby, IR manifolds with direct downhill run from the carby fuel discharge nozzle to the inlet valve. It is hardly required at all with fuel injection with nozzles spraying at the valve.

2) to reduce the need to use rich mixture to start a cold engine for the reasons in 1)

3) To reduce hydrocarbon emissions.

4) To improve response.

5) To reduce the need for accelerator pump shot with carbies.

5) To improve fuel distribution with a central carby.

6) To reduce throttle plate icing.

The various methods impose different costs in manufacture.

The various methods have different impacts on maintenance and durability.

The various methods have different warm up or response rates.

The various methods have different levels of accuracy to which they hold the desired temperature.

Manifold warming is necessary in cold climates but reduces power in warm climates, at high speed and after the engine warms up.

Manifolds designed for maximum power rather than easy starting, emission control, low speed response and economy such as used for racing go to some lengths to insulate or isolate inlet manifolds from heat.

Of course all the above statements are relative.




Regards
Pat
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[scrimshaw]

Hi

thanks to everyone esp. Pat for that very detailed reply.
Some reasons why I get confused -

In my very limited experience with engines I noticed non-coolant manifolds getting pretty hot quickly after start up just by conduction. Won't coolant be the last thing to get to operating temp. therefore be more likely to cool the fuel on a cold start?

As was noted once warmup is reached a cool charge is preferred - is the warm manifold them working against the efficiency of the engine?

How much can a smooth bore inlet manifold really contribute to heating the fuel/air mixture - seems that heat transfer will be pretty limited?

Again I emphasize my limited experience and am just trying to understand the process better and not saying you are wrong.

If I made a manifold without coolant running through it, where previously it had a coolant jacket; what problems would I likely come across apart from start up?

Am I over thinking this and is the manufacturer simply trying to cover all conditions?

Thanks again for your time.

 

[patprimmer]

It evaporates the raw fuel initial deposited on the manifold floor.

The inlet manifold is usually on the other side of the head to the exhaust, so the only close by source of heat is the head which is filled with water. For the head to heat up on the inlet manifold flange the water in the head must have warmed up. With the thermostat closed, the water circulates around the block, head and manifold only so head water temperature and manifold water temperature are similar.

The quickest response is normally by using exhaust rather than water, but it often requires more maintenance and is less controllable.

Many systems use exhaust to heat inlet air before it enters the manifold and water to heat the manifold floor. At low speed the heat transfer is good. At high speed the fact that it is poor does not matter.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[BrianPetersen]

In order that questions were asked ...

1) No; the cylinder head (usually aluminum) gets some warmth in it pretty quickly. As Pat noted, exhaust gives heat faster, but it's harder to design and passages are prone to clogging with carbon. It's also worth noting that if the engine has a catalytic converter, you don't want to be taking heat out of the exhaust stream after start-up - you want the heat to warm up the catalytic converter so that it lights-up as quickly as possible. The coolant also has the advantage that it is thermostat-controlled and cannot get *excessively* hot ... not true of exhaust heat.

2) Yes, manifold heat works against power output once the engine is warm. At higher RPM the effect is small because of little time for heat transfer. Most fuel injected engines don't use any manifold heat.

3) You actually don't WANT to heat the air/fuel mixture itself. You only want to heat up the surfaces where fuel would be prone to collecting on the walls (or in some cases, building up ice). This is another reason why port-fuel-injected engines don't need it, and my experience with carbureted downdraft one-carb-per-cylinder motorcycle engines is that they don't need it, either (carb icing considerations aside).

4) If you run a manifold without coolant heating and the previous design had it, and you want to know what will happen, the answer is "it depends". Maybe nothing will happen. Maybe it will have driveability problems during warm-up. Maybe it will have higher emissions during warm-up because you have to set the cold-idle mixture richer.