On The Road

Letting off some steam

A few weeks before giving a presentation on transportation to my son’s first-grade class, I asked him if he had any questions about cars.

The 7-year-old said he wanted to know “what the gas was in the rear pipe.” I asked him to clarify his question: “Do you mean the liquid gas fill-up pipe or the exhaust gas pipe?” He responded, “The pipe in the back that steam comes out of.”

That gave me an idea for this month’s column.

Before I discuss where the “steam comes out of,” let me provide a basic understanding of an internal combustion engine. After the fuel (gasoline) is filled in the fuel tank, it has to be transferred to the fuel rail. The transfer is executed by pressurizing the fuel with a pump. On today’s modern cars, there are two types of fuel pumps: low-pressure deliver pumps and high-pressure feed pumps.

I could write a complete column on fuel pumps alone, but this time I will just focus on how the fuel arrives at the fuel rail. The fuel rail is a tube that runs across the top of the engine that holds the fuel injectors. The fuel injector’s job is to deliver the pressurized fuel into the cylinder as a mist. There is a setting on your garden hose nozzle called “cone.” The spray pattern of cone is a spherical-shaped mist. This cone spray pattern helps the fuel cover most of the cylinder as a fine mist.

Once the fuel is atomized into the cylinder, the engine can do its job. Most engines are four strokes.

The first stroke is known as the intake stroke. The intake valve opens and lets the atomized fuel into the combustion chamber (cylinder). As the intake valve opens, the piston moves downward, pulling the fuel into the cylinder.

The second stroke is the compression stroke. The valve closes and the piston moves upward, compressing the air and fuel. Once the piston is at the top of the cylinder, the spark plug detonates the air and fuel.

The third stroke is the power stroke. It comes right after the detonation as the piston is driven downward from the explosion.

The fourth stroke is the exhaust stroke. It occurs when the exhaust valve opens and the piston moves upward to push out the exhaust.

Steam stream

So where does the steam come from? A gallon of gasoline weighs approximately 6.3 pounds. It is comprised of 13% hydrogen (H) and 87% carbon (C). A chemical reaction occurs when you burn gasoline using oxygen from the atmosphere. The carbon and hydrogen split, then recombine with oxygen to form carbon dioxide, CO2, and water, H2O. Once it is all done, approximately 7 gallons of water is created for every gallon of gas burned.

This water is in the form of steam or hot gas; when the car is warmed up or running at normal operating temperature, you will almost never see it. But after the car cools down significantly – like after sitting overnight – the water vapor condenses in the exhaust and turns back to a liquid. When you start your car in the morning, the liquid water that has condensed in the exhaust will then come out of the tailpipe as a visible steam. Once the exhaust gets hot enough, it will be invisible again. 

Because this process also contributes to the production of CO2, it is important to make sure your car is in good working order and you follow good driving habits.

Hopefully this answers my son’s question. If you have any questions or suggestions for what I should write about, please email me.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, visit sunnyvaleforeigncar.com, call 960-6988 or email This email address is being protected from spambots. You need JavaScript enabled to view it..

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