Engine Braking

[Christ]

Originally Posted by Arragonis

The trade-off can be difficult to calculate though. If I coast in gear then I use no fuel and with long gearing (30mph / 1000rpm - 6sp Diesel) I can coast for a long time and my MPG reads at infinity. If I coast out of gear I can go slightly further but then engine is burning fuel to keep going and MPG depends on speed, can be between 40-65.

I'm interested in working out whether its better to accelerate more strongly uphill that try to cruise for longer at a lower throttle, and also whether its better to be in a lower gear (less effort but more RPM) vs a higher one. My experimentation so far has not been conclusive.

(All figures Imperial)

It's better to not accelerate uphill at all, at least the way I see it, unless you can't reach your BSFC peak on a flat. Accelerating, regardless of terrain, should be done in the BSFC peak of your engine setup, for the most efficient fuel use per HP production.

In my case, I try my damned hardest not to accelerate on hills at all. Takes too much pedal, and I drive an auto.

[gltux]

Do carburettor engines use fuel when engine braking?
If I understand how carbs work correctly, they work by sucking air through, so if I'm engine braking, does it suck more fuel into the mix or is there a little tap thing to cut it off?
Or have I got it all wrong? I wouldn't be surprised.

[Christ]

Carb'd engines do in fact suck in fuel while engine braking. There are ways to fix this, but no OEM carb that I've seen employs them. The problem with fixing the fuel flow so that DFCO is available is that idle becomes a compromised item. The idea is to introduce a "hat" to the fuel needle when the throttle valve is closed completely, so that some air still gets in, but no fuel enters the stream, as the air doesn't make contact with the fuel needle to draw it out. This interferes with airflow characteristics, as well, and without an on-demand injection system to keep idle, probably wouldn't work too well. The other way that I've seen/thought about to do it, would be to literally cut fuel flow manually by means of a fuel choke in the carb bowl that caps off the fuel jet or the bowl vent. It's not fool proof, but it just might work enough to say so, and won't affect idle or other airflow considerations, only bowl capacity.

[Arragonis]

Originally Posted by Christ

It's better to not accelerate uphill at all, at least the way I see it, unless you can't reach your BSFC peak on a flat. Accelerating, regardless of terrain, should be done in the BSFC peak of your engine setup, for the most efficient fuel use per HP production.

In my case, I try my damned hardest not to accelerate on hills at all. Takes too much pedal, and I drive an auto.

Thanks for the reply.

My quandry (and maybe I should start a new thread rather than take this one to Cuba) is that holding pace uphill in the same gear takes much longer at lower MPG (say around 25-30), but a quick burst in a lower gear takes it down to 20 but only for about 20% of the time and then I can coast for the rest of the way. I have yet to work out which is the better overall.

[dwtaylorpdx]

Originally Posted by Christ

Carb'd engines do in fact suck in fuel while engine braking. There are ways to fix this, but no OEM carb that I've seen employs them. The problem with fixing the fuel flow so that DFCO is available is that idle becomes a compromised item. The idea is to introduce a "hat" to the fuel needle when the throttle valve is closed completely, so that some air still gets in, but no fuel enters the stream, as the air doesn't make contact with the fuel needle to draw it out. This interferes with airflow characteristics, as well, and without an on-demand injection system to keep idle, probably wouldn't work too well. The other way that I've seen/thought about to do it, would be to literally cut fuel flow manually by means of a fuel choke in the carb bowl that caps off the fuel jet or the bowl vent. It's not fool proof, but it just might work enough to say so, and won't affect idle or other airflow considerations, only bowl capacity.

Some German cars from the 80's with webber carbs had fuel cut solenoids, they would close the main when the throttle was closed and vacuum went high enough. they worked great til they didn't.... The solenoids would stick or malfunction... Something I'm sure material tech would fix now...

Dave

[Christ]

I guess it would really depend on how much your MPG drops and how much faster you're going to end up going. Regardless, acceleration requires more than steady state speed does, making it quite obvious that there probably aren't very many scenarios where acceleration uphill would be beneficial. Of course, we've not defined "hill" in this scenario, either.

[bgd73]

Originally Posted by dan765

I'm confused on this. It seems to me that since the engine spins faster while braking, then it is burning more fuel, so how is it more efficient?

throttle closed has air bypass open on injected. the last of the carbs attempted crazy ways to do this, but it is not efficient. it is insanity with an egr valve.

I still avoid engine brake in carb vehicles...and the egr valve may become an air valve sometime, I have been pondering experiments for years.

for injecteds, the engine brake will never be strong because of air valve, but like a rig, at heavy load it is only 5% helper, or wherever a car may be in weight and engine size. still worth choosing and making easy to drive stick shift.

[SentraSE-R]

You could simply close off your carburetor's idle circuit. There's a handy needle valve (aka idle adjustment screw) to do it. Once done, your car would not only shut off when going down hills, it would also shut off automatically at stop signs and stoplights, just like the hybrids.

Of course, there are the minor problems that many times you want to have the engine idling, and you'll waste a lot of gas starting up at high rpms.

[Christ]

Many Mopeds can be setup like that, as well... so you have to pedal to start it every time you come to a stop, and if you get moving too slow, you clutch it and the engine stalls.