P.1 

OAR Box 1830 

Prepped by Ollie Stewart 

Document Number: 

14) IV-B-10 

Docket Number: 

A-87-11 

Printed 11/13/2008 7:20:51 AM Sheet 14 of 232 
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P UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

SEP - 8 1988 

OFFICE OF 

AIR AND RADIATION 

MEMORANDUM 

SUBJECT: 

FROM: 

TO: 

VOC Running Losses from Motor Vehicles 

Richard D. Wilson, 

Office of Mobile Sour 

Air Management Division Directors 

Regions 1, 3, 5, 9 

Air and Waste Management Division Director 

Regions 2 

Air and Toxics Division Directors 

Regions 7, 8, 10 

Air, Pesticides, and Toxics Management Division 

Directors, Regions 4, 6 

Recently the subject of VOC running losses and evaporative 

emissions from motor vehicles has received a lot of public 

attention. The purpose of this memo is to provide you with 

some information which you and your staff can use in responding 

to questions from State and local officials, the press, or 

others. 

Background on Evaporative Emissions 

"Running losses" are emissions of VOC released from points 

other than the tailpipe or crankcase, while an engine is 

running. They are distinguished from "evaporative emissions," 

which are those VOC losses that occur when the engine is turned 

off. Evaporative emissions have been regulated in stages of 

increasing stringency since 1971. The current standards for 

evaporative emissions represent a control level of about 95% 

from uncontrolled. They have been in effect since 1981 for 

light-duty gasoline cars and trucks and since 1985 for 

heavy-duty gasoline trucks and buses. 
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As you know, a few years ago EPA determined that the high 

volatility of commercial gasoline was causing vehicles in-use 

to exceed their certification standard. In August 1987, EPA 

proposed to restrict gasoline volatility. We expect a final 

rule in 1989. We have also found a number of early-1980's 

models that have needed to be recalled for noncompliance with 

the evaporative emissions standard even when tested on the less 

volatile certification fuel. Such noncompliance was not 

surprising, given the stringency of the 1981 standard. Recalls 

to correct these problems are underway and we believe 

manufacturers are doing a better job on their newer models. 

Over the last couple of years we have also realized that 

evaporative emissions are greater at higher ambient 

temperatures than at the temperatures that are used for 

certification testing. The sensitivity of evaporative 

emissions to temperature is greater on current, highly volatile 

commercial gasoline than on certification fuel. For several 

practical reasons we have not proposed to use higher 

temperatures in certification testing. However, less volatile 

commercial fuel would greatly reduce the excess emissions 

attributable to high temperatures in the real world. Also, we 

have been expecting that most vehicles meeting EPA's proposed 

standard for on-board refueling emission controls would have 

integrated evaporative/refueling control systems that would 

control evaporative emissions even at temperatures above the 

official test temperature range. 

Running Losses 

Running losses are nearly all due to evaporation (and in 

extreme cases, boiling) of fuel from the fuel tank. When a 

vehicle is running, its fuel tank temperature tends to rise to 

about 15-25°F above the ambient air temperature. This heating 

occurs because of the hot air flow from the engine compartment 

down under the vehicle, from the proximity of the hot exhaust, 

from hot pavement, and from heat generated in the rear axle 

bearings and gears. On fuel injected cars, fuel is constantly 

circulated from the fuel tank to the engine compartment and 

back, and the returning fuel carries significant heat back to 

the tank. U.S. vehicles are designed so that any vapor 

generated in the fuel tank is carried by a hose to the engine, 

where it is inducted and burned. When the engine is off, this 

same hose carries vapors to the carbon canister for storage. 

The various hoses and valves are arranged so that a single 

vacuum line to the engine draws vapor from the fuel tank and 

purges the canister with fresh air. 

Officially, running losses with standard test fuel at 

standard test temperatures during the standard 20 minute 

driving cycle must be counted against the evaporative emissions 

standard. However, under these conditions running losses are 

believed to be virtually zero on new vehicles. Consequently, 

in practice running losses are not measured on certification 

prototypes. 
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With high volatility fuels and high ambient temperatures, 

a faster vapor flow can be generated in the fuel tank than is 

inducted into the engine through the purge line. The excess 

flow in the vapor line enters the carbon canister. Once the 

carbon is saturated with vapor, the excess flow is released to 

the atmosphere. It is also common under these conditions for 

fuel tank pressure to increase so much (since a flow restrictor 

in the vapor hose to the engine creates a back pressure) that 

venting occurs at the filler cap. As a safety measure, filler 

caps are designed to release any pressure in the fuel tank 

greater than about 0.5 to 2 psi above atmosphere. 

Vehicles vary in their design rate of purge flow into the 

engine, and in how quickly fuel in the tank heats up. In 

addition to ambient temperature, cloud cover, and wind speed, 

the driving pattern affects the rate of temperature rise in the 

tank, at least on short trips. Driving pattern affects air 

flow under the vehicle, fuel recirculation, axle bearing heat 

rejection, heat rejection from the engine, and exhaust pipe and 

muffler surface temperatures. The result is that vehicles' 

running losses at any instant depend on the vehicle geometry, 

trip speed, trip history, fuel volatility, initial fuel tank 

temperature (if higher than ambient due to a previous trip or 

to solar warming), and ambient temperature. 

The running loss value of 9 gram/mile which you may have 

heard about or seen in the trade press is an average for the 

first five vehicles we tested. It applied only for the 

combined extremes of trip length (60 minutes), temperature 

(95°F), and volatility (Reid Vapor Pressure of 11.5 pounds per 

square inch). Several more recently tested vehicles have had 

lower average running losses. For comparison, the exhaust HC 

standard is 0.41 gram/mile and a car just meeting the 

evaporative HC standard would emit the equivalent of about 0.13 

gram/mile. 

To date, the Office of Mobile Sources has tested about 10 

late model vehicles for running losses, most of them at 6 

combinations of temperature and fuel volatility. We are 

planning to test a few older vehicles and some light-duty 

trucks over the remainder of this fiscal year. We also plan to 

test some cars with different driving cycles. The first 

quarter of FY 1989 will be devoted to data analysis, prior to 

release of MOBILE4 in January or February. The large 

variability in test results from vehicle to vehicle will cause 

a larger uncertainty in the MOBILE4 estimates than we would 

like, but we see no way out of this predicament. 
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Implications for SIP's 

The effect of including running losses in M0BILE4 win be 

to increase base year VOC inventories. However, we are 

proceeding to a final rule on reducing fuel volatility, to 

which running losses are very sensitive. If the proposed RVP 

limits are adopted in the final rule, running losses will be 

controlled by about 80 to 85 percent. This would greatly 

reduce the impact of running losses for inventory years after 

the effective date of the RVP reduction. This date is expected 

to be before the attainment date targets in EPA's proposed 

post-1987 policy and in the various bills before Congress. 

Subsequent phase-in of the additional reduction from vehicles 

equipped with better control systems will assist maintenance. 

While the direction of the impact on base year VOC 

inventories is clear, its size is not. Running losses as high 

as the 9 gram/mile cited earlier occur only on long trips at 

very high temperatures, e.g., 95°F. It will take more work to 

develop methods for State and local planners to estimate the 

value of running losses for their local ozone situation. It may 

be that running losses are in many cases not very important to 

peak ozone levels because local travel patterns are such that 

most running losses occur too late in the day to react fully. 

Running losses could be more important in multiple day episodes 

and precursor transport situations. 

There has been some speculation that running losses are 

the "missing inventory" that accounts for lack of progress to 

date on ozone reductions and/or contradictions between the 

inventory ratio of HC to NOx and the ambient ratio. Such 

speculation is premature and certainly is no basis for relaxing 

efforts to improve other parts of the VOC inventory. Also, 

while running losses may well be a significant portion of an 

area's VOC inventory on a hot day, they may be very much less 

important on cooler ozone violation days. 

I am not attaching any test data since the test results to 

date would be more a source of confusion than enlightenment and 

because we are adding vehicles to the data base each week. 

However, if there is any other way we can be of assistance to 

your office in this area between now and when MOBILE4 is ready 

for release, please let me or Charles Gray in Ann Arbor know. 

cc: Gerald Emison 

Don Clay