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Discussion Starter · #1 ·
The original thread contained a mostly jocular repartee on the ability of the Prius to clean the air.

This post runs the hard numbers on carbon monoxide.
I think we will all be surprised.

First we will consider the Prius on idle in the enclosed garage.
Second we will examine the undiluted exhaust.

For reference, we need the toxicity of carbon monoxide (hereafter CO).

The EPA describes >15 ppm CO as poor air quality.
The highest CO level recorded in the US (was in Yellowstone Park) in 1996 was 36 ppm.

From the MSDS:
200 ppm: Slight symptoms (headache, discomfort) after several hours of exposure.
400 ppm: Headache and discomfort experienced within 2-3 hours of exposure.
1,000 -2000 ppm: Within 30 minutes, slight palpitations of the heart occur. Within 1.5 hours, there is a tendency to stagger. Within 2 hours, there is mental confusion, headache, and nausea.
2000-2500 ppm: Unconsciousness within 30 minutes.
>2500 ppm: Potential for collapse and death before warning symptoms are produced.

[Here I will use ppm by weight. CO has the same molecular weight as N2, so ppm by volume is not significantly different.]
1. Prius on idle in enclosed garage:
--The Prius is SULEV. Take the SULEV standard for carbon monoxide of less than 1 g/mile.
--The Prius EPA mileage is 48 mpg.
--To yield 48 g CO per gallon of gasoline consumed.
--The Prius tank is 11.9 gallons.
--The Prius will idle for about ten days or about 1.2 gallons per day.
--To yield 58 g CO after idling for 24 hours.

We need a garage.
Take a two-car garage of 20 X 20 X 10 ft for 4000 ft3 or 113 m3.
Air weighs 1.275 kg/m^3 for 144 kg (144,000 g) of air.
From above, 24 h of idle yielded 58 g CO.
Thus, 400 ppm (58/144000) CO are produced after idling the Prius for 24 hours in a 2-car garage.

If we spend 2-3 hours in an enclosed garage in which the Prius has been idling for 24 h, we will get a headache.

Practical Application: While not advised, <1 h cabin warmup on idle in an enclosed garage should not pose major health concerns. After 1 h, roughly 17 ppm of CO will be released. This is just above the EPA threshold of poor air quality (>15 ppm) and less than half of the maximum 1996 US value (36 ppm).

2. Undiluted Prius Exhaust:
--From above 48 g CO produced per gallon of gas used.
--Gas weighs 6.1 lbs per gallon.
--A stoichiometric burn is 14.7 lbs air to 1 lb gas.
--That is, a pound of gas produces 15.7 pounds of exhaust.
--One gallon gas (6.1 lbs) produces 95.8 lbs exhaust.
--Or 48 g CO per 95.8 lbs (43,450 g) exhaust.

--Or 1105 ppm CO in pure Prius exhaust.

So if you breathed undiluted Prius exhaust from car forced to run its gas engine for two hours nonstop you will experience mental confusion, headache, and nausea. Not death.

The Prius will not reduce CO levels in Philly or anywhere in the unenclosed US (1996 max 36 ppm, Prius exhaust 1105 ppm).

Practical Applications (1) after a very, very bad day: Suicide by gas pipe appears impossible using the Prius. Does provide a cooling-off period for the depressed, however. Hopefully, this does not promote driving the wrong way down the freeway as an alternative.

Practical Applications (2): On farms, feral cats are used to reduce vermin. They proliferate and must be culled. A common and humane way to cull is to collect the cats in a burlap sack and tie the sack to the exhaust of a car. The Prius is not the car for this grim, but necessary, task.

Conclusion: There seems little chance that the Prius can generate lethal CO levels.

PS The above is a best faith analysis of Prius exhaust based on published data. No liability is accepted. The Surgeon General has determined that breathing pure Prius exhaust does not improve your sex life.

· Registered
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Discussion Starter · #7 ·
EPA CO numbers

Since Doug did such a great job finding the emissions data I thought I would update the original post in this thread that used the SULEV CO limit as a maximum for the Prius.

However just getting the tab delimited data is a real nuisance. So for now I will simply restate the EPA values with a few comments. I put the CO g/mile in red. The first column is the state (CA or the rest of the country NL), then something called useful life that I believe corresponds to expected emissions of an older car (mileage in thousands), the test procedure used, CO, the standard (e.g., SULEV=1 g/mi), and the standard name (e.g., sulev).
Sales Area U Life Emission Cert Level Standard Tier
CA 50 CO-COLD 1.8 10
NL 50 CO-COLD 1.7 10
CA 120 CO 0.34 1 SLEV
NL 100 CO 0.18 2.1 ULEV
NL 50 CO 0.12 1.7 ULEV
NL N/A 50-CO 0.4 1.7 ULEV
CA N/A 50-CO 0.4 1 SLEV
CA 4 CO-US06 0.09 8 LEV
NL 4 CO-US06 0.09 8 LEV
CA 4 CO-SC03 0.13 2.7 LEV
CA 4 CO-SC03 0.52 2.7 LEV
NL 4 CO-SC03 0.13 2.7 LEV
NL 4 CO-SC03 0.52 2.7 LEV
Remember that I used <1 g/mi and Doug used 0.34 g/mi (row 3).

Note that the newer (50,000 mile) car, if I understand this right, will emit only 0.12 g/mile (which may help explain Robert's comments).

On the other hand, the cold engine/cold cat gets 1.7-1.8 g/mi for reasons that are explained in the link found in the chemistry of combustion post on the general forum, something I ignored in my ballpark figures as a brief departure from the norm.

The SC03 entries with varying results were otherwise identical in the original and they are included for completeness.
Idle Time Full-Tank
My original calculations depended on an estimate that the full-tank Prius would idle for 10 days (actually 237 hrs) was from Sam Williams here:

Julian estimated 8.5 days (205 hrs) while charging a depleted 12 V battery here:

The full-tank idle time may be strongly dependent on ambient temperature.
I asked Doug about hydrocarbons, but I forgot I used to know the answer as detailed in this neat snippet.
[T]he cleanest motorcycles in the 2001 model year
certified at a level of about 0.32 grams/mile of hydrocarbons on the
Federal Test Procedure. On the exact same test procedure the Prius
certified at a hydrocarbon level of 0.0024 grams/mile. The cleanest
(highway) motorcycle is therefore more than 100 times dirtier than
the Prius.

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