How can flex-fuel vehicles use gas and ethanol?

Q

uestion:

I would like to hear your opinion on flex-fuel vehicles. I assume these can use regular gasoline with 10 percent ethanol or E85 (85 percent ethanol).

I don't understand how a vehicle can run on both gasoline and E85 when we hear of potential engine damage when using E15 (15 percent ethanol).

Answer: That's an issue that involves technology, engineering, materials, politics, economics, and regulations. I'm going to focus on the economics for the vehicle owner and the engineering and technology that allow using different blends of ethanol and gasoline.

First, recognize that E15 is supposed to be compatible with vehicles using regular gas and E10. Flex-fuel vehicles can operate on any ethanol-blended fuels.

Internal-combustion engines prefer 100 percent gasoline. There's a reason the industry ended up choosing it as the primary motor fuel nearly 100 years ago, and that is energy content per unit.

Modern technology, engineering, and materials make today's engines capable of operating on ethanol-blended fuels with reasonable efficiency and success. Fuel tanks and fuel systems, valves and valve seats, pistons and piston rings, and other components of flex-fuel vehicles are designed to handle higher percentages of ethanol without issues.

A fuel-compensation sensor or discriminator identifies the gasoline/ethanol specific composition, allowing the powertrain control module to adjust fuel flow and timing to properly burn the fuel. From an operational and performance view, the two fuels are relatively seamless. Drivers won't notice a difference.

Building flex-fuel vehicles allows carmakers to offset potential penalties for not meeting corporate average fuel-economy standards. Ironically, many flex-fuel vehicle owners are unaware their vehicles can run on E85. The energy and emissions associated with each type of fuel continue to be debated.

Perhaps the biggest question for flex-fuel vehicle owners is this: Which fuel provides the lowest cost-per-mile operation? Because of ethanol's lower energy content per unit, it takes more E85 to drive a mile than the same vehicle operating on E10 or E15.

Thus, the price at the pump is the key. Is the difference between E85 and regular gas/E10 enough to offset the loss in miles per gallon?

Q: My car is a 1998 Nissan Sentra SE with almost 150,000 miles. The temperature gauge falls abruptly toward cold and flutters whenever the engine is over 2,500 r.p.m. Head gasket, right?

This condition appeared almost a year ago, and the car still starts and runs perfectly. No loss of coolant; no odor and no coolant contamination; no coolant in the oil. The heater worked fine all winter.

Could there be another reason for this that's not serious and a backyard mechanic could fix?

A: Yes, a bad coolant temperature sender or gauge. I agree that if there were a serious issue with a head gasket, you'd be aware of it by now (overheating, coolant loss, etc.).

Low coolant level could cause this symptom as air bubbles flowed by the sender. It's possible something is aerating the coolant. Try starting the engine cold with the radiator cap off and watching as it warms up. Any serious combustion leak into the coolant will tend to cause the coolant to bubble out of the radiator.

The simplest fix is to apply black tape over the gauge and check the coolant level regularly.