Re: Quit Being A Goddamn Idiot, Bill Hughes!!

When induction air is pulled through the venturi it increases density at the top of the restriction. This causes an increase in heat, but this heat is given up when liquid fuel is added to the air stream. at the bottom end of the venturi, where the throttle butterfly lives, the cooled airstream decreases in density, and cools to a temperature lower than it was at the beginning of the journey. Any water vapour that was in the air condenses onto the throttle butterfly, and if the temperature is low enough, it will turn to ice.

This process can occur at any altitude, in any season. All that is needed is a set of temperature, humidity, and throttle conditions that are conducive to icing, and you've got it. Typically though, it occurs on a set throttle which is only partly open - exactly what occurs in aircraft. In extreme conditions though, icing will occur during changing loads and throttle openings, such as in an automobile.

The freezing typically begins on the engine side of the throttle butterfly. Ice gradually accumulates there and eventually closes or at least restricts the gap between the butterfly blade and the barrel of the carburetor. The result is loss of power and a stuck throttle, not nice in an aircraft. Corrective action is to pull the carburetor heat lever, which brings hot air either into the induction airstream or else directly to the venturi. This causes a further reduction in power until the ice has gone, then power increases to a point where carburetor heat can be reduced, which further increases the power up to full power without heat.

You don't need to be at high altitude for this to happen, but it is certainly just as common an occurrence at high altitude as it is at sea level in, say, equatorial New Guinea.

I've only ever flown turboprop/turbofan aircraft, but icing even in low humidity/low altitude conditions can be a problem for those too (air vortices off fan blades, etc. expand and contract a lot). Even flying around North Florida in the cold part of the winter, we'd have to crunch the numbers for humidity, density, etc. Can't comment on carbed engines, but it must be worse.

Matt

Asking Bill to stop being an idiot is a bit like asking your dog to stop licking his butt. The dog will look up and acknowledge the command, but pretty soon his butt will itch again, and he will go back to cleaning it.

The best you can do is hope he doesn't lick your face next ...

And, Bill - like the dog - might be annoying but he means well is right more often than not, except when he drifts completely off target. Your dog drifts off target too, but then you scream at him to stop licking his nuts.

freezeups.

Yes, all venturis cause a temperature drop which can cause carburetor icing. _Atmospheric water_ is what freezes. It's especially prevalent in Lycoming and Continental engines because the updraft carburetor is on the bottom of the oil pan. It's rarest on American V8s because the carburetor is (was) on the manifold where it stays hot. Carburetor icing occurs at any outside temperature ABOVE freezing, but usually between 70 and 40 degrees Fahrenheit and in high humidity.

Below 32 degrees OAT carburetor icing doesn't occur.

This is on the Private Pilot written test as you pilots know. It's why you apply carb heat in the pattern. Carb icing is most problematic with aircraft engines but can occur on inline racing car engines with SU or Weber carbs with fresh air piped in so it isn't breathing hot underhood air, or on tractors with updraft carbs out in the breeze.

Propane is a vapor, not a liquid,when it enters the carburetor. So if the propane carb has a venturi it could ice. But it just doesn't seem to be a common problem.

So were the old super-charged, piston-engined fighters of WWII more immune to this (i.e. because of the heat imparted to the air charge entering the engine)? I swear I thought some of those piston-engined bombers of WWII flew regularly at altitudes above 20,000 feet.

-Bill (remove "botizer" to reply via email)

They didn't have carburetors so I would imagine carb icing wouldn't be much of an issue, even if in a different form. I've flown fuel injected aircraft and there's no carb heat knob to pull.

Jerry

Well, the earlier Rolls Royce Merlin powered fighters and bombers were certainly carburetored, and suffered from icing, so much so that the counter - measures were positively scary. Basically, an adjustable gate in the exhaust pipe from one of the cylinders was opened, and the exhaust flame was piped to a sleeve around the carburetor.

This is perhaps a little less scary than that used in the Tiger Moth (Gypsy Moth and such - there are plenty of them still flying today). In these, the exhaust gate allowed the naked exhaust flame to play onto the outside of the venturi pipe. Funnily enough, engine fires in these types were relatively common! Safer methods were installed into most of the versions still flying, but there are some that retain the naked flame principal.

The earlier supercharged engines had the supercharger between the engine and the carby, so icing would certainly be a way of life with them. These planes achieved extreme altitudes though, as the superchargers were fitted with 3 speed gear boxes and a clutch. Before going into the next altitude range, the engineer declutched and changed into the appropriate gear! The Mosquito fighter/bomber was fitted with these superchargers, and its combat altitude ranged from zero to 40000. The Flight Engineer/Navigator was a very busy person in these planes!

I'm not certain about the later superchargers fitted ahead of the carby though, but I would bet that icing would rear its ugly head somewhere.

I thought the Spitfires were all direct fuel injection? I thought this was one of the big deals about the Merlins is that they were fuel-injected?

-Bill (remove "botizer" to reply via email)

No British or American liquid cooled production engines had direct FI. The Daimler-Benz (the company was Daimler-Benz:the automobiles were called Mercedes Benz)DB6xx inverted V12s were. They used a Bosch diesel type pump ruggedized to handle nonlubricious fuels and diesel style nozzles, directly into the combustion chamber. They were not diesels: they had spark plugs and intake throttles and were operated in pure SI mode. Later, people figured out that with Diesel injection systems and spark plugs you could build multifuel engines, but the Benz aeroengines, along with the 300SL inline six and the 300SLR/W196 Grand Prix desmo valve straight eight,were not diesel,"semidiesel", or multifuel.

The early mark RR Merlins had float bowl carburetors, later mark RR Merlins (and AFAIK all Packard built engines) had a pressure carburetor.

There are probably several Web sites, and various books and periodicals that cover this stuff.

As Hershel Smith and other authors have pointed out, there were various Diesel aircraft engines and toward the end of piston aircraft engine development by real companies (P&W,RR,etc: not Lycoming or Continental, they couldn't develop a piss-up in a brewery) that was the trend. The US military resisted the trend for the stated reason that they didn't want to provide two fuels at air bases, with serious consequences if a misfueling took place. The early jets offered so much more speed the military had to accept them even if a different fuel was required, but in fact, the early jets were all sold to the Navy and the Army Air Corps on the basis that they could burn either avgas or kerosene. When the military found that using kerosene enhanced safety, provided extra range for a given gallonage, and didn't require frequent lead defouling, jet fuels were here to stay. But the Navy ran its carrier jets on avgas for several years so as not to carry two fuels.

Jet engines on mixed-power aircraft-the P2V Neptune and the B-36-ran on avgas their whole careers. That's not to say modern gas turbines would tolerate leaded fuels at the ITT they run today.

True. This led to possibly the only advantage enjoyed by the Germans during Battle of Britain dog fights. Because the British Hurricanes and Spitfires used float bowl carburetors, the aircraft had to be flown in a positive g mode at all times, otherwise the engine would stop. This meant (for example) that the aircraft had to be rolled before entering a dive, whereas the Germans FI engines did not. This split second delay in evasive action was responsible for a lot of German kills.

God, you are stupid. Really, breathtakingly stupid.

Detroits have racks (pre-DDEC). There are no more two-stroke Detroits in truck service and a scant few in commercially operated buses. All four-stroke Detroits are DDEC except the 8.3 V8 which is almost extinct as well.

Cumminses have PT injection, with a unit injector actuated by cam but fuel control regulated by rail fuel pressure(Or CELECT, which is the electronic version.) The B Series engines have Bosch pumps and I'm not sure what the ISBs use.

These are diesel systems and they aren't used on spark ignition engines at all.

Aircraft FI is indirect and continuous flow on all engines built in the USA except certain R-3350 Wrights. The two basic port injection systems are the Continental-which is similar to the Hilborn on drag cars-and the Bendix/RSA which is most like K-Jetronic in the auto world. It's mostly found on Lycomings.

I was just reading a Ballantine books publication about the P47 Thunderbolt in which one of the aces of the P47 stated that the Germans quick dive evasive reaction didn't work against Thunderbolts. The Thunderbolts didn't use FI did they?

-Bill (remove "botizer" to reply via email)

Most Brit aeroengines with carb heat AFAIK had the design of running engine oil through a carb passage, heating the venturi. A much more sanitary practice than the US carb heat boxes which car manufacturers in the 70s emulated for cold emissions compliance. I'm going by the inline engine of a Auster AOP a local guy owns (sadly, he listened to the hangar queens and converted it over to a piece of s*it Lycoming-but he still has the original FWF crated...)