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Generator Emissions

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I've been taken to task for a somewhat dismissive response to a recent question or comment about genset APUs (Auxiliary Power Unit - this is a power generator in, or towed by the vehicle). So I thought that I would amend that response and provide some possibly useful information. Rather than repeating what's available in the archives already, however, I thought it might be worth adding some data to the discussion.

[edit] Reducing Emissions

While I recognize that minimizing emissions isn't an important consideration for all EV hobbyists, many of us are interested in being clean or "green." Others may find the comparison of at least academic interest.

We've said here before several times that an EV with an APU genset produces appreciably greater emissions than an equivalent ICEV. However, seldom does anybody cite actual numbers. So, with a fair bit of handwaving and many (probably unfounded) assumptions, I've put together a rather questionable but perhaps useful comparison between 3 flavors of ICEVs that EVDL members might own, and a light passenger car conversion (Honda Civic, Geo Metro, etc.) EV powered by an APU.

Any corrections for errors that I may have made are most welcome.

First, let's look at EPA passenger car and light truck emissions standards.

Tier 1 EPA limits (fully applicable to 1997 model year vehicles, 50k mile warranty required) :

 NMHC (nonmethane hydrocarbons) + NOx (gasoline) = 0.65 g/mi
 CO = 3.0 g/mi

Tier 1 ULEV EPA limits (from 2001, as I understand it, mandatory in CA and voluntary in 49 states; I don't know which vehicles meet this standard but they are probably all passenger cars and not "light" trucks) :

 NMHC + NOx = 0.24 g/mi
 CO = 1.7 g/mi

And, for those who want to compare the cleanest ICEs (such as 2004 and later Toyota Prius), PZEV / light duty SULEV-II EPA limits :

 NMHC + NOx = 0.03 g/mi
 CO = 1.0 g/mi

Now, let's calculate our genset's emissions. We make several assumptions.

1. Our genset was made after 2000, when EPA's second tier regulations on small spark-ignition engines for portable equipment took effect. (Might have been 1999, I'm not sure I got that right.)

2. Our genset was made by a reputable manufacturer. It's not some cheap knockoff purchased at a tool fair for $300, and thus the manufacturer has made an effort to meet EPA standards.

3. A small, well-designed passenger car conversion EV requires about 10kW to travel a constant 60 mph on flat ground. Thus our genset can produce 10kW continously.

4. We are traveling at a constant 60 mph.

5. The genset is always operating at full load.

6. We neglect any effects of the genset's additional weight or aerodynamics on the efficiency of the EV.

7. We neglect losses from rectification of the genset's output and in the EV's charger and other electronics. The 10kW figure cited above is taken as the entire sum of the input.

8. Generator efficiency is 75%. On the net, I found a Honda genset with an 11hp engine (8.2kW) which has a rated full load output of 6kW. From this I deduce (possibly erroneously) that the generator in this genset is about 75% efficient.

As I said, lots of assumptions. Corrections would be welcomed.

EPA rates non-handheld engines for general duty service - including portable generators - not in grams of emissions per hour, but in grams per kiloWatt- hour. This is kiloWatt hours of >>engine output<<, not electrical output. Applying our 75% efficiency to the above 10kW, we find that our genset's engine has to produce a continous 13.3kW to keep the car moving at 60mph.

Now, let's compute our genset's emissions from traveling 1000 miles at 60mph.

EPA Tier 2 standards for non-handheld engines > 225cc are :

12.1 g/kWh HC + NOx
11.2 g/kWh NMHC + NOx (note 1)
549 g/kWh CO

Note 1 : I couldn't find the NMHC figure on the EPA website; this number came from Canadian standards, which the document claimed are harmonized with US EPA standards.

To travel 1000 miles at 60 mph we will have to run our genset for 16.7 hours. We are using 13.3kW of engine output * 16.7hr = 213kWh from engine.

Engine emissions per 1000mi =

 11.3 g/kWh * 213 kWh = 2407g NMHC + NOx
 549 g/kWh * 213 kWh = 116937g CO

Dividing by 1000 we find that on average, in this very strictly defined situation, our clean, EPA qualified genset produces

 2.4 g/mi NMHC + NOx
 117 g/mi CO

Compared to a pre-2001 passenger car (or most "light" trucks), operating at the very limit of EPA standards, our genset-driven EV produces :

 270% more NMHC + NOx (3.7 times as much)
 3800% more CO (39 times as much)

Compared to a more recent Tier 1 ULEV passenger car, the genset-driven EV produces :

 900% more NMHC + NOx (10 times as much)
 6800% more CO (69 times as much)

And compared to a very clean passenger car (PZEV-SULEV II, such as 2004+ Toyota Prius), the genset-driven EV produces :

 7900% more NMHC + NOx (80 times as much)
 11600% more CO (117 times as much)

Now, some thoughts on fuel consumption. The assumption has been that a genset will yield poor returns in the MPG department. This may in fact turn out to be true, when all factors are considered (especially the extra weight and/or aerodynamics hit of the genset). But ignoring those figures, we find something interesting.

I found this page which gives specs for a 9.7kW genset with a Honda GX-610 engine:

http://www.jobsite-generators.com/multiquip_portable_gas_generator_ga_9.7hz.html

It weighs 349lb (!) and runs for 6.75 hours on its 10 gal gasoline tank. That's roughly 1.5 gph. The page doesn't specify whether this is at half load or full load, but let's give them the benefit of the doubt, and assume for the moment that it's at full load.

If so, our 16.7 hour, 1000 mile trip will require 25gal of fuel, for an FE of 40mpg! This compares surprisingly well with the 48mpg overall I get with my 1995 Honda Civic VX ICE.

Of course, in the real world this calculation may fall apart. Add the weight and aerodynamics hit (350lb on a trailer will affect a small EV), the possibility that an actual "10kW" genset couldn't supply that much power for several hours straight, and possibly a less efficient engine type - and it's hard to say what the real world mpg would be.


David Roden - Akron, Ohio, USA EV List Administrator

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