Vapor pressure will vary

Q In your article last week about lower fuel mileage during winter, you mention some things I never thought of. I don't know for sure, but have heard that there is a winter fuel and a summer fuel provided by the oil companies. The summer octane is higher to reduce the likelihood of pinging. The winter octane is lower to ignite for easier starts. The annual average octane is what is shown on the pump sticker.

I have long attributed some of the difference in fuel mileage to the difference in octane. Is this true?

A You've got the right idea but the wrong fuel characteristic. Octane ratings don't change from summer to winter, but the gasoline's vapor pressure does. The easiest way to understand vapor pressure is to recognize that when a liquid's vapor pressure is equal to atmospheric pressure, it boils. The best example is boiling a pot of water. When the water's vapor pressure reaches 14.7 pounds per square inch, with typical atmospheric pressure, the water converts to steam.

Why is vapor pressure important with gasoline? Because engines cannot burn gasoline in liquid form. The fuel must first be atomized, broken into tiny droplets of liquid, then vaporized and mixed into the incoming air stream.

In summer when temperatures are high, gasoline with vapor pressure approaching atmospheric pressure can boil well before it reaches the induction system on your engine. This is "vapor lock," and it's long been a problem with summer driving.

Conversely, in winter when temperatures drop below zero, too low a vapor pressure may prevent the fuel from vaporizing at all, meaning it remains in liquid form and wets combustion chambers and spark plugs. This is a "flooded" engine.

So, federal and state standards have been established for gasoline vapor pressure, based on the Reid vapor scale, which measures vapor pressure at 100 degrees. In Minnesota, standards call for a maximum Reid vapor pressure (RVP) of 9.0 in winter to aid cold starts, and a maximum RVP of 7.8 in summer to minimize potential vapor lock. Ethanol-blended fuels are allowed a 1.0 higher vapor pressure.

The problem is only in the morning and never happens during the day. Battery, cables and connections are OK. When I turn the key to start, all dashlight symbols come on but the temp, clock and radio lights go out.

A This is a classic example of high electrical resistance somewhere in the starting circuit. As you continue to try the key, the connection heats up because of the resistance, expands a bit and finally completes a solid enough connection to pass the necessary current to crank the engine.