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Old 10-08-2008, 09:56 AM   #1
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slower route while engine warms up

Hello all

I have a theory i would like to share and ask if it's complete BS or not.

My regular route to my gf's house has 2 ways of starting, from my driveway i can go straight onto the motorway or i can go through town about 2-3 miles and come out the other side and join the motorway that side. I am convinced driving at city speeds (30mph) for 2-3 miles while the engine heats up saves me petrol. I haven't seen any concrete numbers yet, its just my suspicions.
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Old 10-08-2008, 10:08 AM   #2
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Interesting question.

On the FI systems I've seen, enrichment is by way of an increase in injector pulsewidth, so to minimize losses I guess you would want to have as fast of a warmup as possible, with as low of an engine speed (i.e. top gear). In other words, take the highway route and shove it into overdrive.

However I believe some FI systems will modify the target A/F ratio during warmup, which would make RPM irrelevant, and would only make the amount of time until warmup relevant. In this scenario the highway would still be better because you would warm up faster (higher engine load) and get into "normal" (closed loop, 14.7:1 AFR) mode sooner.

Of course I'm just blabbering with no hard statistics, so I'd be curious to see you do a few tankfuls of each method and see if you can see some meaningful results.

-Bob C.
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Old 10-08-2008, 10:19 AM   #3
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the whole reason i am theorising this is because my car does better FE while at city speeds, so I figure i ,ayaswell take advantage of that while the engine warms up..
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Old 10-08-2008, 10:23 AM   #4
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Does your city route have any stop signs or traffic signals? Or do you maintain a fairly constant speed?

Speeding up and slowing down is generally much worse than maintaining steady highway speeds.

-BC
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Old 10-08-2008, 11:10 AM   #5
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bobc, you're right about FI warmup. To add some detail, cold enrichment is richest at low loads and tapers off so there's less cold temp enrichment at higher loads. This is why I try to avoid idling with a cold engine. I just get in and go, although I make sure to drive gently until the engine is halfway warmed up.
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Old 10-08-2008, 11:17 AM   #6
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My route always goes to a stretch of road that starts at 45 MPH and goes ro 55 MPH before I travel 7 tenths of a mile.

If you drive the local route make your engine work while its cold so you get the most out of the energy losses while warming up.

I start it and go. I think thats the best way to warm up the engine, use the high idle to get the job done. My car is usually warn in less than 1.3 miles, guage starts moving in less than 1 mile.

A cold engine produces oil pressure within 2 seconds of initital start.

I had a 37 Ford that would get 60 PSI on the original 6 volt system cranking the engine at 100 RPM, ignition off in a matter of 5 seconds.

regards
gary
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