Preventing Vapor Lock with Ethanol Blending

Imagine you are driving your car and the motor suddenly stops. Hopefully this doesn´t happen on a crossing. While trying to restart the car, the hot motor stops again. Even the breakdown service can't find any problem with the engine on connecting the car to their computer.

The car is towed away and the manufacturer is contacted, who gives many suggestions and sends spare parts. Perhaps, the manufacturer sends them for free, as they are concerned about solving the issue with their car. However, even they cannot find the issue. Finally, a completely new motor is purchased, but the issue still might not end there. The only solution seems to be purchasing a new car.

Every year, across the world, several thousands of cars face similar issues. This happens due to the gasoline vapor lock that causes engines to stop, typically when the car is in traffic and the temperature is increasing. In the same way, when a car is parked for a short time period, the heated motor will not start. With increasing temperature and altitude, it is more likely that gasoline vapors develop a vapor lock, blocking the fuel in the line from moving to the engine.

Regulations to Prevent Vapor Lock

Since gasoline vapor pressure is also necessary to evaluate fuel performance and the risk of outgassing, the U.S. Environmental Protection Agency (U.S. EPA) imposes stringent regulations for vapor pressure on petroleum manufacturers. To avoid vapor lock and environmental problems, automotive fuel specification ASTM D4814 demands gasoline to be tested as per ASTM D5188. This standard controls the evaluation of vapor-liquid ratio temperatures. In automotive fuel specifications, the temperature at which a ratio of 20:1 (vapor:liquid) is attained (generally known as T(V/L) = 20) signifies the risk of the fuel to develop a vapor lock. This risk is higher in hotter climate and in higher altitude.

How is Ethanol Blending Affecting Vapor Lock Tendency?

Testing fuels for the risk of vapor lock formation is of renewed interest since petroleum refineries are adding increasingly more ethanol to gasoline, and since the entire world faces longer and longer periods of hot weather. Several older engines simply have not been developed to be prepared to match modern fuels. In the United States, increasingly more states are adopting the E10 rule, which permits 10% ethanol (EtOH) to be added to the fuel.

The US EPA often has to modify gasoline volatility regulations for the petroleum industry, to avoid emissions becoming very high and to ensure drivability. Furthermore, this article will also show that even new cars are affected.

The major reason for why it is important to test fuels for their V/L ratio temperature is due to the fact that ethanol blending impacts the vapor pressure. The vapor pressure of ethanol is much lower when compared to that of gasoline. The addition of ethanol varies the temperatures at which different V/L ratios occur. Adding 10% of ethanol to gasoline — as is presently done in the United States — considerably decreases the V/L ratio temperature. Thus, a critical vapor-liquid ratio, which can lead to vapor lock, will be attained at lower temperatures with gasoline-ethanol mixtures when compared to regular gasoline. Illustrated in Figure 1 is the temperature-dependent V/L behavior when various amounts of ethanol are mixed with gasoline.

Temperature changes at various V/L ratio for 1%, 5%, 10%, and 15% gasoline-ethanol blends. Highlighted is T(V/L) = ~35.

Figure 1. Temperature changes at various V/L ratio for 1%, 5%, 10%, and 15% gasoline-ethanol blends. Highlighted is T(V/L) = ~35.

Interestingly, there is no linear solution on how ethanol mixtures impact the vapor pressure. Figure 1 exhibits an unexpected behavior of gasoline-ethanol blends: Based on the V/L ratio, the temperature curve surprisingly changes its direction and moves up rather than moving down. This “S” curve phenomenon can be observed for E5 and E10 mixtures, but not for E15 mixtures. Another phenomenon can be observed in Figure 1: For low V/L ratios, the T(V/L) is higher for E15 than for E10 blend. Both curves cross each other at a V/L ratio of 35:1, caused by the “S” curve phenomenon for E10.

Although these results may be surprising, there is no need to worry about blending ethanol into gasoline. However, this example shows how necessary it is to evaluate fuels not only at a V/L ratio of 20 but also over the FULL T(V/L) range — as mandated by ASTM D5188 — to find out about the vapor pressure behavior of modern blended fuels.

Performing tests at T(V/L) = 20, as presently done for ASTM D4814 automotive fuel specifications, is just not providing adequate information about fuel vapor behavior. This example also suggests that blending higher ratios of ethanol with gasoline as performed at present might lead to a more predictable behavior of blended fuels.

How About Modern Injection Motors and Vapor Lock Tendency?

Furthermore, vapor lock is a rising issue for car manufacturers, who are producing new engines, which must run on various fuels. When modern injection motors break down after traveling for a long distance, some might not be able to restart right away. This is because those new motors reach temperatures of up to 120–140 °C, and vapor lock is likely to occur at such high temperatures.

Specifically, in the case of modern fuel mixtures, vapor lock issues occur more often. Recent feedback from car manufacturers implies that they have a definite need to identify vapor pressures of up to 5–6 bar and to closely observe vapor pressure during the production of their new generation engines.

How to Test Vapor Lock Tendency?

Established globally as “The vapor pressure specialist,” Grabner Instruments has built the MINIVAP VPXpert, an innovative, very versatile analyzer to evaluate temperature at various V/L ratios for ASTM D5188 tests. Moreover, the MINIVAP VPXpert is the optimum solution for testing vapor pressure of gasoline-ethanol mixtures at high temperatures. It enables ramp measurements up to 120 °C, offering the highest temperature range of vapor pressure testers available in the market.

Working with the world-renowned Grabner vapor pressure test method, the analyzer conforms to all industry vapor pressure testing standards, thus allowing safe development of the new-generation automotive engines for ethanol-blended fuels.

MINIVAP VPXpert—Expert vapor pressure testing at highest temperatures

MINIVAP VPXpert — Expert vapor pressure testing at highest temperatures

This information has been sourced, reviewed and adapted from materials provided by Grabner Instruments.

For more information on this source, please visit Grabner Instruments.


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