Carburetor Icing, Manifold Pressure, Superchargers, and Water Injection
Half the reason for my absence over the last few days - apart from having a bad cold, and wrestling with an internet service provider that thought VirtualFlight.Online was a walking denial of service attack - is that I’ve been finally learning my around the PMDG Douglas DC-6 in Microsoft Flight Simulator.
If you’ve not looked at the DC-6, and you’re at all interested in how radial engines work, or even how a single pilot (you) might attempt to fly an aircraft that sometimes had as many as six crew, then you’ve been missing out.
I warn you now - and I know I often joke about it - the DC6 is likely to induce post traumatic stress disorder. It’s by far the most involved and immersive simulation of a historic aircraft in Flight Simulator.
Anyway.
While getting my head around the various controls within the cockpit of the DC-6, I found myself falling down rabbit-hole after rabbit-hole, refreshing my memory about a number of aspects of engine management. Sometimes in Flight Simulator an aircraft might require knowledge of one or two of them - the DC-6 requires all of them.
Where to start?
Carburetor Icing
Carburetor icing is a sneaky problem that can occur even on seemingly warm days. It all boils down to two key factors: temperature drop and moisture.
There are two main culprits behind the temperature drop:
Venturi Effect: The carburetor has a narrow section called the venturi. As air speeds up through this constriction, its pressure drops according to Bernoulli's principle. This pressure drop also causes a cool down effect, similar to how a can of compressed air gets cold when you release the pressure. This cooling can be significant, bringing the temperature down by as much as 70°F (40°C).
Fuel Vaporization: When gasoline mixes with air in the carburetor, it rapidly changes from a liquid to a vapor. This vaporization process absorbs heat from its surroundings, further lowering the temperature inside the carburetor.
Now, with the air and carburetor itself being quite cold, what about the moisture?
Ambient Moisture: Air always contains some moisture, and as the temperature inside the carburetor drops, this moisture can condense into liquid water droplets.
Warm and Humid Conditions: Warm air holds more moisture. So, even on a seemingly warm day, if the humidity is high, there's more moisture available to condense and freeze in the cold carburetor.
If the temperature inside the carburetor dips below freezing, these condensed water droplets turn to ice. This ice can accumulate on the throttle valve and other internal parts, restricting airflow and causing engine performance issues.
Here's a quick summary:
Colder air due to venturi effect and fuel vaporization
Moisture from ambient air condenses on the cold surfaces
Ice forms and restricts airflow in the carburetor
This is why carburetor icing is a concern for pilots, especially during descents in cool, moist conditions. Luckily, most carburetors have a "carb heat" function that allows warm air to be routed into the carburetor to melt any ice buildup.
Manifold Pressure
Combustion engines rely on air intake to create power. This air intake is measured by manifold pressure, which is the pressure of the air entering the engine cylinders. At altitude, the key issue is the air itself.
Here's why altitude affects combustion engines:
Thinner Air: As you go higher, the air thins out. There are fewer air molecules per unit volume. This means there's less oxygen available for combustion in the engine cylinders.
Reduced Pressure: Along with fewer molecules, the overall pressure of the air also decreases with altitude. This lower pressure reduces the force pushing air into the engine's intake manifold.
Manifold Pressure Limitation: Because of the thinner and lower pressure air, the engine struggles to suck in enough air to achieve optimal combustion. This limited airflow restricts the engine's ability to create power, and it becomes manifold pressure limited.
Imagine a naturally aspirated engine (without a turbocharger) like a large syringe. At sea level, the syringe plunger easily pulls in a full dose of dense air. But at altitude, it's like trying to fill the syringe in a thin atmosphere - you might not be able to pull in the same amount, limiting the engine's potential.
This is why high-performance aircraft engines often use turbochargers or superchargers. These devices compress the incoming air, essentially forcing more air into the engine at altitude, overcoming the manifold pressure limitation and maintaining power.
Superchargers
The DC-6 utilizes superchargers for a critical reason: to maintain engine power at high altitudes. Here's how it works:
Thin Air, Less Power: As the DC-6 climbs, the air thins out. This means there are fewer oxygen molecules available for combustion in the engines. With less oxygen, the engine struggles to produce its full power.
Supercharger to the Rescue: The supercharger acts like an air pump for the engine. It compresses the incoming air at lower altitudes, forcing more air into the engine cylinders. This compressed air has a higher density, essentially packing more oxygen molecules into the same space.
Maintaining Power Output: By providing denser air, the supercharger counteracts the effect of thin air at altitude. This allows the DC-6 engines to maintain closer to their sea-level power output across a wider range of altitudes.
Here's an analogy: Imagine a naturally aspirated engine (without a supercharger) like a large syringe. At sea level, the syringe easily pulls in a full dose of dense air for combustion. But at altitude, it's like trying to fill the syringe in space - you wouldn't get enough air. The supercharger acts like a compressor on the syringe, allowing it to draw in a sufficient amount of air even at high altitudes.
In essence, superchargers are crucial for the DC-6 to achieve and maintain its desired performance, especially when operating on routes that take it to higher altitudes.
Water Injection
The DC-6 used water injection, also sometimes called Anti-Detonation Injection (ADI), for a different reason than superchargers. Here's the breakdown:
Superchargers for Air Density: As you mentioned, superchargers address the issue of thin air at altitude by forcing more air into the engine. This combats the power loss due to reduced oxygen availability.
Water Injection for Detonation Control: Water injection tackles a separate problem - detonation or "knocking" in the engine cylinders. This can occur even at sea level under certain conditions.
Here's how water injection helps:
Cooling Effect: Water injected into the intake air stream absorbs heat during the compression process. This cooling effect helps prevent the air-fuel mixture inside the cylinders from reaching excessively high temperatures.
Detonation Prevention: Extremely high temperatures can cause the air-fuel mixture to ignite prematurely in the cylinder, leading to detonation. This is a much faster and uncontrolled burn compared to normal combustion, creating a knocking sound and potentially damaging the engine.
Leaner Mixture, More Power: With water injection keeping things cooler, the engine can run on a slightly leaner fuel mixture (more air, less fuel) without risking detonation. This leaner mixture can actually lead to a slight increase in power output.
Key Points:
Superchargers address thin air at altitude for maintaining power.
Water injection combats detonation for smoother and potentially more powerful engine operation.
Not Mandatory, But Situational:
It's important to note that water injection in the DC-6 wasn't mandatory for every flight. It was typically used in specific situations that could lead to detonation, such as:
Hot Days: High ambient temperatures could increase the risk of detonation.
Heavy Takeoffs: When the aircraft is heavily loaded, more power is needed for takeoff. Water injection could help achieve this power safely.
Short Runways: Taking off from a shorter runway might require maximum power, and water injection could be used for a temporary boost.
Overall, water injection in the DC-6 provided an extra layer of control and power optimization, especially during demanding operational scenarios.
The Automated Flight Engineer
If all of this talk about the DC-6 trying to destroy itself in so many and varied ways has disuaded you from looking at it, it also has a rather neat trick - an automated flight engineer (let’s call him “Gary”). As soon as you switch him on in the tablet, he will prepare the aircraft for you completely automatically - busying himself around the cockpit, telling you what he’s doing as he does it.
There’s just one problem with Gary - he doesn’t always make the right decisions. It’s worth knowing a bit about what he’s up to - hence my reading - before entrusting him to your passengers lives.