Why no fast acceleration?

[serialk11r]

Originally Posted by oldtamiyaphile

You mean on sports and or turbo cars.

My (turbo) Fiat will do enrichment pretty early. My 2014 Renault is very reluctant to go into enrichment and my old school cable throttled Jeep has heaps of power below 2000 and won't do enrichment at all unless road speed is over 50mph.

That's a Jeep though, I was thinking more like Corolla. My MR2 had a 1ZZ-FE and even pulling a 2200lb car, it groans loudly but doesn't really go below 2000rpm. Newer economy cars can go open loop at lower rpms these days too. The manufacturer is incentivized to make the engine run rich if they don't need the excess power while running government tests since the consumer is the one paying for the extra fuel, and it helps keep the replacement rates on the cats under warranty down.

I also suspect that a Renault (who doesn't sell cars in the US) sold in Australia would have a different fueling map than a US market car for regulation or cost reasons.

Now that you bring a counterexample up though, I feel like I need to go put the OBDII scanner on the other cars in the family and see...heh.

[PaleMelanesian]

My Fit has electronic throttle and it doesn't go into Open Loop enrichment until the pedal is basically on the floor. I had to be more careful to avoid it with my old cable-throttle Civic. It really depends on the specific design.

[CFECO]

In the past, small gasoline engines, carbureted, the best fuel economy was always full throttle and coast. "pulse and glide" as told here I guess.

[H-Man]

Originally Posted by CFECO

In the past, small gasoline engines, carbureted, the best fuel economy was always full throttle and coast. "pulse and glide" as told here I guess.

Depends on the car. My maths that that my dad's corolla could get 55-65 MPG while drafting a truck at 60 MPH and it was a 1.6L carbed engine with a 4 speed lockup TC only in 4th. ~2300 RPM at 60 MPH in 4th with locked TC. Math was needed due to stop and go traffic at the end of trip, I assumed pre-2007 city fuel economy since the traffic was bad enough to resemble it

[skyking]

If the target speed is going to be optimal, say 45~50 MPH, the sooner you can start reaping the benefit of that speed the better, IMO. Diesel favor a more brisk acceleration for this reason. Poking along with the beetle in slow acceleration just prolongs the agony of the scangauge being down in the ~30 MPG range. I can go briskly at ~20 MPG instant, for a few seconds. Then I'm cruising at ~60 MPG at those optimal speeds.

[BV1]

Similar thread: http://ecomodder.com/forum/showthrea...ate-28617.html

I'll share my thoughts from it with a few updates:

As an a instructor and a tuner this is something I get asked alot.

The short an simple answer is that disregarding air resistance or losses due to great average speed, the ideal eco-acceleration should be the maximum acceleration available prior to entering open loop, or pulling ignition timing due to knock, or increasing AFR, in the highest gear possible (or lowest RPM). And... that acceleration is only useful if upon reaching the intended speed one immediately starts to Pulse and Glide, at least in a 5spd. This is because the gear ratios are locked for a particular gear, so in order to take advantage of the higher efficiency near WOT one must provide a lower RPM return, via the P&G technique.

I tend to accelerate under heavy load in 5th gear (as soon as its reasonable to take), but reduce the acceleration enough to stay in closed loop.

This minimizes frictional losses in the engine by using the piston stroke, valve duration and pumping losses to their maximum ability and since rotational losses are not load dependent the greater power output increases the efficiency of each engine power stroke by reducing the loss %.

Its the same idea as having to withdraw $400 from a ATM, would you do it all in one hit or in 10 little hits? Your gonna get dinged $1.00 for each transaction so might as well minimize the number of transactions by maximizing the available amount in each one.

Its the same reason why the same engine in a turbo diesel variation can get better mileage then in that engine in non turbo diesel variation. The extra RPM needed in the non turbo causes unnecessary frictional losses.

If you want to give your brain a stretch on this material, take a look on these 1960 tests on tractors, in particular, HP-Hr/Gallon, and note the better efficiency at higher load, of course these did not have a WOT enrichment device on them, so the most efficient engine is the one that took the biggest advantage of the combustion stroke. Think of the HP-Hr/Gallon as comparable to KiloWatt Hours/Gallon on an electric generator, if you can get more power (KwH) out for the same gallon, your doing better.

TractorData.com Oliver 88 tractor tests information

Hope that helps,

Steve

[oldtamiyaphile]

Originally Posted by CFECO

In the past, small gasoline engines, carbureted, the best fuel economy was always full throttle and coast. "pulse and glide" as told here I guess.

Not if they had an accelerator pump...

[mr bad example]

Originally Posted by skyking

If the target speed is going to be optimal, say 45~50 MPH, the sooner you can start reaping the benefit of that speed the better, IMO. Diesel favor a more brisk acceleration for this reason. Poking along with the beetle in slow acceleration just prolongs the agony of the scangauge being down in the ~30 MPG range. I can go briskly at ~20 MPG instant, for a few seconds. Then I'm cruising at ~60 MPG at those optimal speeds.

i find this true in my smart, we call it "drive it like you stole it". accelerating for 8-10 seconds at 1.8-2 GPH uses less fuel than accelerating 1.3-1.5 GPH for 15-20 seconds. (i use an ultra gauge) of course these figures are for smart fortwo's, your consumption may vary by make and model

[superrick]

well we know that fast starts and fast stops does consume more fuel, so it would make sense to accelerate at the most fuel efficient speed.

[coverturtle]

I would say some testing needs to be done. It's too easy to generalize and what may be the best way to drive one car may not be true for another.

In my reading about car engine design, I was surprised to find that the design point for car engines is 85% throttle. This means that car engines are, or were, most efficient at 85% throttle. That's probably volumetric efficiency. With the advent of computer control, this may no longer be relevant. From a theoretical viewpoint, the factors we want to look at are instantaneous fuel consumption, speed and aerodynamics. It's relatively easy to hook up some sensors and measure or calculate fuel consumption, speed, distance covered and acceleration over a test run but not so easy to figure in the negative force from air resistance. Finding the most fuel efficient way to drive a car may require a number of test runs, but if the test runs do not give consistent results, it may be problematic.

Recently I drove my car over 200 miles at freeway speeds and found that the mpg of my next tank of gas around town was much better than before the run to the big city and back. I attributed this to the distance run "cleaning out the engine" or burning up the combustion byproducts such as carbon buildup, temporarily making the engine more efficient until the small errands around town builds up the deposits again. It reminded me of earlier days when I lived with racers who would change the jets in the carburetors to give the best air-fuel mixture. The criterion at the time was the color of the deposits in the exhaust pipe. Black was too rich, nearly white was dangerously lean and gray was ideal. How gray? That was a matter of experience and the color had to examined at the end of a good long run. For some that was just 10 miles on the freeway; for others it was at least 30 miles.