From: John De Armond
Subject: Re: Gas consumption at idle?
Date: Sat, 12 Aug 2000 19:17:16 -0400
bill horne wrote:
> GBinNC wrote:
> > Somebody asked this question recently, but it went unanswered, so I'll
> > try it again and expand on it:
> > Does anybody know how much gas a Dodge 318 engine consumes at idle
> > (with a/c running) -- or where I can find out?
> > Also, I've always known that idling for a long time isn't good for an
> > engine. But what is considered "a long time"? Five minutes, fifteen
> > minutes, an hour?
> > What I'm trying to figure out is this: in a case where I need a/c for
> > just a short time (while waiting in a parking lot for my wife to buy a
> > few groceries, for example), is it better to idle an already hot
> > vehicle engine and use the dash air, or start a cold generator to use
> > the roof air? (Either a/c will do an adequate job in my Class B.)
> > The generator already gets regular exercise, so that's not relevant
> > here. (What I'm talking about isn't long enough for proper generator
> > exercise anyway.)
> > Factors to consider are the length of time an engine can idle without
> > negative consequences, the amount of fuel consumed, the amount of air
> > pollution created by an idling V-8 engine (compared to a 2.8 Kw Onan),
> > whether it's good for the generator to start and run it for just a
> > short time, etc.
> > Any advice, or even an educated guess?
> > GB in NC
> From various sources on the web:
An amazing list of irrelevant sites.
> A typical car uses about a quart per hour.
This is an undated japanese anti-vehicle site. The number is
irrelevant to modern, closed loop emission controlled engines.
> Passenger cars idling produce 371 grams/hour of CO in winter, and 229 in
This is the output of an EPA computer model (GIGO) and is supposed
to represent the composite emission rate across the entire US fleet.
Ignoring the fact that it's a model estimate, this is irrelevant to
a specific emission-controlled engine.
> The 'automotive industry' says that 1 hour of idling causes as much wear
> as 2 hours of normal operation.
You cite a BBS message as authoritative? Wow. This is absolutely
false for EFI'd engines. The engine runs as cleanly at idle as it
does at cruise and since the mix doesn't have to be enriched like it
does on carbureted engines to achieve a smooth idle, the engine is
not subjected to oil thinning, cylinder wall washing and all the
other things that harm carb'd engines. The reality is that with an
EFI'd engine, the specific fuel consumption remains remarkably
uniform over the entire RPM range.
> Personally, I see no reason for 200 million vehicles to add to the
> pollution problem by creating unnecessary pollutants just to keep the
> vehicle cool while shopping. Those in PU trucks can roll down the
> windows, or sit on the tailgate. Those in vans and bulgemobiles can turn
> on the fantastic Fantastic Fan that one hears so much about in here.
The fact of the matter is, a modern EFI'd engine, running in closed
loop control emits essentially nothing. what used to be a joke is
now fact - that in high smog areas under bad conditions. the exhaust
is actually cleaner than than the air going in. The reason for this
is simple - the emission standards have gotten so silly that
emission control systems are designed to the specifics of the
emission test. Even with the engine emitting essentially nothing
when warmed, the brief emissions during the first few seconds after
cranking can consume most or all the emission budget of the test
cycle.* Other "terribly important" (not!) things such as how much
vapor can diffuse through the walls of fuel system components now
dominate emission engineering.
GB, to answer your specific questions, if it matters, undoubtedly
the generator will emit more since it's carbureted and without a cat
converter. As for fuel consumption numbers, I suspect that it will
be a draw. Certainly the BSFC of the Onan is much worse than the
318 but it's a much smaller engine. It would take measurements of
your specific system to know for sure. If I know your idle speed,
manifold pressure and injector pulse width, I can compute the fuel
consumption rate exactly. I'd have to put a flowmeter in the fuel
line of the generator.
In terms of damage to the 318 from idling, that will depend on how
well your van cools at idle. The engine operates as if it is at
cruise, only at 600 rpm. EFI systems are designed to inject the
next cycle's fuel just as the intake valve closes for the previous
one. This lets the gas lay on the valve and evaporate until the
valve opens again. What little gas remains is atomized by the
almost supersonic flow through the valve opening as it first lifts
off its seat. What enters the cylinder is gaseous fuel. there is
no liquid gas to wash down the cylinder walls. And the mix is
controlled to precisely stochastic by the oxygen sensor so there is
no carboning. The only real issue that remains is how well does the
cooling system work at idle?
My advice is to use whichever system is more convenient. I run my
generator when stopped because it does a better job of cooling than
does the dashboard system.
* We (or y'all, since I don't buy new vehicles) will be paying big
for this silliness. In order to eliminate the emissions of the
first 15 seconds or so of operation, such crazy "solutions" as
battery heated catalysts (the major impetus to moving to 42 volt
battery systems), molten metal heat storage systems and other even
crazier systems are to be deployed in the next few years. Purchase
cost is only part of the issue. Maintenance cost is the rest.