There are many reasons why an engine might fail to start, one of the most common being vapor lock. Vapor lock occurs when the fuel in your fuel lines vaporizes, turning from a liquid to a gas. Vapor lock can disrupt the delivery of liquid fuel to the engine, resulting in the aircraft stalling. If an aircraft stalls, it may no longer be able to produce lift and may be more difficult to restart once it has been warmed up. With this in mind, this blog will provide a brief overview of vapor lock and why it happens in order to understand how you may prevent it.
First, we will begin by looking over what a fuel-injected engine system layout looks like. Within a fuel tank, there are filters, sumps, and fuel pumps. Moreover, there are several fuel lines that branch out from the center to meet several cylinders in the system. Vapor lock is likely to occur in the fuel injector lines as the fuel is being delivered to the injector nozzles in order to be sprayed into the intake manifold. The heat from the engine cylinders and crankcase makes its way up to the fuel injector lines.
Beyond vapor lock causing the aircraft to stall, fuel that burns in your engine produces air pockets that can get stuck inside the fuel lines. The greater the volatility of fuel, the likelier that vapor lock will occur. Avgas 100LL is a gasoline specially designed for piston engine aircraft and is typically less volatile. Volatility is the measure of how likely a substance will evaporate at a particular temperature. Interestingly enough, high volatility makes it easier to ignite an engine in normal conditions, but can cause vapor lock. In the case that there is too little volatility, the engine may not start.
Vapor lock normally happens when you shut down an engine. When the engine shuts down, heat rises up to the fuel lines. Without air traveling through the cowling, the fuel in the lines begins evaporating. However, if you experience vapor lock when you try to start your plane after a sharp turn, shut down the engine and restart it in about 30 minutes. In rare instances, vapor lock may also occur while in flight.
Vapor lock can be minimized if you begin by shutting down the engine and opening the cowling or oil vents. Alternatively, if you can turn your aircraft toward the wind, the breeze can reach the cowl and into the engine to cool it down. If you have tried everything to keep your engine cool, and vapor lock still happens, you can try to push the air bubbles out of the fuel lines by injecting large amounts of fuel through with a boost pump. As there is no clear indicator to let you know that all the vapor has been driven out, success comes with observation and experience.
You can restart your engine by running the fuel pump for about 5-10 seconds and try to start your engine up again. Repeat these two steps until your engine starts up, and if it does not, check to see if fuel is dripping onto the ramp below the engine. In the case that it is, the engine may be flooded, which can make restarting more difficult and can increase the odds of an engine fire.
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