Carburettor heat in Spits and Hurri

[IvanK]

Originally Posted by Sutts

Is it true that the merlin had inbuilt and continuous carb heating? Any period refs you can point me to please?

The reason I doubt this point is the same reason carb heat is usually controlled by a switch/lever in the cockpit - heating the air going into the carb reduces engine power as warm air is less dense than cold air so you get less air going into the cylinders to burn with the fuel. This is the same reason compressed air from the supercharger is often cooled before it gets to the carb (to remove the heat created during compression).

Now I can't see the designers of the merlin permanently reducing engine performance by continuously heating the air entering the carb.

On the other hand, if just the carb body is heated then I suppose the effect on the air might be insignificant. Would be nice to see some docs on the subject.

Cheers

The Merlin variants as modelled in COD are NOT subject to carburettor icing thats why no Carby heat or controls are present... they are not required.

How can this be ? Well the designers thought about it right from the word go. The SU twin barrel carburettor throats are jacketed. The Jackets around each carburettor throat were fed with Hot return flow engine coolant before it got to the radiator. This ensured that the Carby throats (and therefore butterfly) were always warm and prevented the formation of carburettor icing. See jpg of the carby. Item 18 shows the jacketing and warm fluid flow as the purple shading. The mounts for the Internal butterfly valves were also fed with warm scavenge oil shown in red.



From this Pub:



When rough running in cloud was discussed with the devs they indicated that it was caused by Propeller icing. Though IMO more work on conditions conducive to icing needs to be done in COD. In addition if we are seeing propeller icing then Airframe icing should be an issue as well ... it isnt at the moment. We see Ice build up on the canopy but not the airframe.

[NZtyphoon]

Some Spitfires and other British aircraft had an oval mesh ice shieldto stop ice ice accumulating at the mouth of the carby intake, either restricting airflow or breaking off in chunks; it also stopped ice from the prop - best image I have is of a Mk XII. It generally knocked off a few mph.

[Sutts]

Originally Posted by IvanK

The Merlin variants as modelled in COD are NOT subject to carburettor icing thats why no Carby heat or controls are present... they are not required.

How can this be ? Well the designers thought about it right from the word go. The SU twin barrel carburettor throats are jacketed. The Jackets around each carburettor throat were fed with Hot return flow engine coolant before it got to the radiator. This ensured that the Carby throats (and therefore butterfly) were always warm and prevented the formation of carburettor icing. See jpg of the carby. Item 18 shows the jacketing and warm fluid flow as the purple shading. The mounts for the Internal butterfly valves were also fed with warm scavenge oil shown in red.



From this Pub:



When rough running in cloud was discussed with the devs they indicated that it was caused by Propeller icing. Though IMO more work on conditions conducive to icing needs to be done in COD. In addition if we are seeing propeller icing then Airframe icing should be an issue as well ... it isnt at the moment. We see Ice build up on the canopy but not the airframe.

Thanks Ivan, Tomcat for this great info. I had no idea such a clever design was built into the carb.

I stand corrected.

[Blackdog_kt]

Originally Posted by Sutts

Is it true that the merlin had inbuilt and continuous carb heating? Any period refs you can point me to please?

The reason I doubt this point is the same reason carb heat is usually controlled by a switch/lever in the cockpit - heating the air going into the carb reduces engine power as warm air is less dense than cold air so you get less air going into the cylinders to burn with the fuel. This is the same reason compressed air from the supercharger is often cooled before it gets to the carb (to remove the heat created during compression).

Now I can't see the designers of the merlin permanently reducing engine performance by continuously heating the air entering the carb.

On the other hand, if just the carb body is heated then I suppose the effect on the air might be insignificant. Would be nice to see some docs on the subject.

Cheers

I was thinking about this too. Running on warmer (meaning, rarified) air means less air volume for a given fuel volume. It's just like running on too rich a mixture = power loss.

Or does the system just heat the carb from the "outside", resulting in simply warming the "tube" and not the air in it?

[SlipBall]

[QUOTE=Blackdog_kt;395419]I was thinking about this too. Running on warmer (meaning, rarified) air means less air volume for a given fuel volume. It's just like running on too rich a mixture = power loss.

Or does the system just heat the carb from the "outside", resulting in simply warming the "tube" and not the air in it?[/QUOTE]


Yes, that is true...basically like a radiator tube as found in your car, or home effect see #18/24 in dia...would be curious to know if there was a control lever for restricting flow

[Skoshi Tiger]

It would also ensure that any liquid fuel (from the accelerator pump) that hit the carburettor walls would vapourise quickly giving better throttle response.

Until I saw that diagram i didn't know that they had an accelerator pump. I didn't think they were that common in aero engines.