Why no fast acceleration?

[jamesqf]

Originally Posted by oldtamiyaphile

Open loop does = enrichment.

That's strange. Admittedly the Insight is not a "normal" car, but the only time I see it going into open loop (other than briefly on a cold start) is when coasting in gear with zero load, or even regen. The MPG bar is always pegged at 150 mpg when this happens (though it can be pegged in closed loop, too), so I always thought it meant the engine was in DFCO. Enrichment with the fuel cut off doesn't make sense, does it? Or is this just Insight weirdness? (It's the only thing I've ever driven with a ScanGauge or similar.)

[ProDigit]

It seems obvious that getting as fast as possible into final gear is the key to best fuel economy, for as long as the RPMs aren't too low, and the engine isn't lugging.

When in last gear, the slower you accelerate, the better the MPG, because the slower the speed will be.

I think when talking about acceleration, it's the process between a departure from standstill, to that final gear; and what the best form of acceleration is.
For instance, suppose you'd only be going 40MPH.
What is the best MPG for acceleration?
Is it 1st, 2nd, 3rd, 4th, 5th, 6th?
Or could it be 1st, 3rd, 6th?
How deep should the pedal be pressed, at what RPM should the engine be kept?

And what if coasting at 40MPH in 6th; and one wants to accelerate to 80MPH.
Does one get better MPG shifting into 5th, or keep on accelerating in 6th, even when the acceleration is very fast?

I think those are the most important questions to ask and see answered.

[oldtamiyaphile]

Originally Posted by jamesqf

That's strange. Admittedly the Insight is not a "normal" car, but the only time I see it going into open loop (other than briefly on a cold start) is when coasting in gear with zero load, or even regen. The MPG bar is always pegged at 150 mpg when this happens (though it can be pegged in closed loop, too), so I always thought it meant the engine was in DFCO. Enrichment with the fuel cut off doesn't make sense, does it? Or is this just Insight weirdness? (It's the only thing I've ever driven with a ScanGauge or similar.)

In the context of this thread (accelerating) open loop = enrichment.

However, what open loop really means is that the ECU is ignoring the O2 sensor(s). This also happens on engine start and during DFCO events.

[oldtamiyaphile]

Originally Posted by ProDigit

I think those are the most important questions to ask and see answered.

Look at my vehicles owned side bar - 2,4,5,6 cylinders, petrol/diesel/hybrid, NA/turbo, 0.9-4.0L, 900-2000kg, 0.33-0.45CD. I can assure you that each of them is quite different. Although guidance from this forum is great, you've just got to work it out for yourself in the end.

[Astro]

I am a WOT fan for several reasons.

I travel in peak hour traffic a lot and find getting ahead of the traffic leaving the lights but not catching up to the next group of traffic works really well.
I get a whole section of road to myself where i can choose my speed and not have to contend with other drivers pulling in front of me and jamming their brakes on causing me to brake hard as well.

Then there are the traffic lights. If i accelerate slowly, then i seem to catch the red lights more often. It feels like the lights are sequenced for travelling at or near the speed limit. Once you are caught by one red light, if you don't get a move on when it turns green then you will get caught a couple of lights further down the road.

Then there is the sanity aspect. Travelling in a crowd of drivers who spend more time fiddling with the radio, texting, even reading while driving puts an unnecessary strain on my nerves.

I will happily travel in the relative serenity between clumps of traffic.
It is also nice to know that the WOT style isn't hurting my mileage.

[oldtamiyaphile]

Originally Posted by Astro

It is also nice to know that the WOT style isn't hurting my mileage.

Except that it is. How much over EPA are you running?

[PaleMelanesian]

Look at the big picture. Acceleration makes a difference in mileage, but overall speed and minimized braking and situational awareness affect mileage much more. Just use moderately high throttle to accelerate, with lowish rpm, and you're most of the way there. The focus on coasting, no-brake-ing, timing traffic lights and such. Acceleration is important, but it's just one of many facets to hypermiling.

[Saturn1994]

What I have found is that best fuel economy is based on load. I haven't got a good method for measuring load (manifold pressure is one way). However, I do have a rule of thumb, and I have it somewhat automated: I manually accelerate to 30 or 40 mph, and then engage cruise control. If on a fairly level surface, I can press the + button every 1 second on my 1994 Saturn SW1 SOHC, which will give me fairly smooth 1 mph acceleration every second (i.e. to go from 40 to 60, pulse the button 20 times). If on an incline, I need to increase the time between pulses depending on how steep the incline. What I try to avoid is pushing the ECM to drop gear and accelerate the engine, and to avoid loading the engine so that the ECM enriches the fuel mixture.

[JohnAh]

Originally Posted by BV1

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

...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). ...upon reaching the intended speed one immediately starts to Pulse and Glide,... ...gear ratios are locked for a particular gear, so in order to take advantage of the higher efficiency near WOT...

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 %.

I have two quite different experiences that verfy all this about engine load, rpm's and BSFS-maps:

The first is an experiment started in pure desperation when my little Fiat 127 got a conrod bearing seizure. I brutally ripped out the faulty rod/piston and their closest counter-acting neighbour. I stick-welded the lubrication orifices in the crank and grinded down the corresponding cams. The once 1050cc engine became a 20-25 hp 525cc with 180 degree crank. During the following year I drowe the little car 30.000 realy hard, with a lot of high revs on lower gears to keep the same average speed as before. Despite the hard running I got a 10-15% reduction of fuel consumption. I guess it's a good example of reduced friction and pumping losses, as well as a proof of the benefits of higher load. Driving the car with downsized engine was sometimes a bit scary in the beginning since I was used to engine braking to reduce speed. With only half the engine left it almost felt like the car had a freewheel.

For some time I thought that engine downsizing was the great quickfix to chop off a chunk of fuel costs. The reduced engine power may however be both stressful and even dangerous. The rev-range is also reduced a lot and you cal almost count the piston strokes while driving on low revs. The need of high revs to get the necessary power counteracts some of the fuel savings. So no, engine downsizing is not a perfect quickfix, but it's a quite cool thing to have tried!

Projektblogg - Fiat 127 engine downsizing

A much more rewarding quickfix is to change driving style. -Enter the mighty P&G, Pulse and Glide, or BURN & GLIDE as i prefer to call it! After the little Fiat I have a 1975 Saab 96 V4. Owners of such cars are used to a consumption of about 7-8 L/100km (33mpg). This summer I made around 4,3 L100 (55mpg) again and again, with slightly reduced average speed but extreme use of P&G, or B&G as I prefer to call it. The method is truly about finding the BSFC sweetspot and staying there while the engine is used. If you can't stay within the sweetspot, the engine should be SHUT OFF! I got realy good fuel savings from DWB, Driving Without Brakes, and even more when I introduced B&G. When I finaly introduces engine shut-off I saved even more.

Using B&G with engine shut-off together with DWB is both fun and hard work. It's also extremely annoying to other drivers during rush hour and dense highway traffic. -Use the method responsible! Since it's a very activ driving style you become more aware of other cars and how they interact. Getting skilled with B&G+DWB in city traffic can actually help you get where you want quicker than the average driver! You learn to "read" traffic in a totaly different way. I often pass the same angry BMW several times, making him even angrier, being passed by the same slow and scrappy Saab again and again... -The harder they try, the harder they fall!

I try to avoid revs over 3000. I try to keep the engine around maximum torque at 2500 rpm and perhaps 70-90% of full throttle to avoid carburettor enrichment. I also add about 20-25% ethanol to the fuel. This servers three purposes: Ethanol is cheaper, makes the fuel-air ratio a bit leaner (closer to the carburettor's margin) and prevents pinging, premature ignition and keeps the engine from running backwards (!) every time I shut it off after each heavy burn-cycle. I've also heared that ethanol burns cooler and slower than petrol, which can improve fuel efficiency at lower revs.

High revs are bad because of increased friction and pumping losses, low revs are a bad match between piston speed and heat transfer to the engine block. -Just take a look at a BSFC-map or even a rpm/torque graph and you can clearly see that fuel efficiency is best where the torque is around maximum.

[JohnAh]

Originally Posted by ProDigit

Curious about a Wankel engine...

And I'm curious about a basic two-stroke, when it comes to extreme ecodriving by Burn & Glide. The most basic two-strokes with piston control induction, no variable exhaust port and no fancy fuel injection is in great need of distinct pressure waves to give maximum power and something you may (at best call) fuel efficiency. My unproven theory is that such a simple two-stroke have a much steeper drop in their BSFC-map below WOT. So perhaps a vintage car with a two-stroke engine can have an even larger inprovement than a 4-stroke by performing B&G/P&G. I hope to try this when my Saab 96 1964 replica is finished (which may happen any year now...)

The Wankel is an interesting design, partly related to the two-stroke engine. They are infamous for high fuel consumption, which I think comes from the long and narrow combustion chamber that gives a poor ratio between surface and volume. (high heat loss) I think the Wankel's comparable to a regular 4-stroke piston engine when it comes to pumping losses. I don't know how they compare with friction losses, but I guess their efficiency may drop steeper on low revs because of heat energy losses from the poor combustion chamber. So perhaps a wankel can be more efficient than a 4-stroke piston engine on higher revs? At realy high revs I guess the wankel also can benefit from the lack of restricting valves. -When the ports are open, they ARE realy wide open!