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Old 12-30-2020, 01:36 PM   #1 (permalink)
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How do you get engine braking with fuel use? Timing Retard?

I drive an electronic throttle car, and unfortunately below 1500rpm, if you come off throttle, there will be no fuelcut, if you come completely off throttle, TorquePro shows timing going quite retarded, (maybe -12 degrees) and there is still quite a substantial engine braking effect

At the lowest possible throttle position (quite easy to do on an electronic throttle) i can have the same fuel use, and throttle position, but with +21 degrees advance, and a small amount of torque from the engine, an improvement from negative.

The fuel use parameter in Torque is not always accurate (it is during warmup), but is skewed with EGR operation, as Torque assumes all airflow is fresh air and does fuel use based on that (i believe)

So, in these off throttle events, what is going on? Fuel being kept around stoch mixture (14.7:1 AFR) at/near the minimum throttle position? Is the timing purely to reduce torque to a minimum? Or is the mixture going lean and this is because of knock concerns? At roughly 1400rpm off throttle, Torque Pro shows fuel use to be roughly double that of idle, despite the engine producing negative/no torque.

I know that in fuelcut operation (openloop, -15 timing), the EGR and throttlebody are commanded to let some extra air through, and increase manifold air pressure (reducing effective vacuum)
and coming out of fuelcut, the throttle closes and pressure decreases.

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Old 12-30-2020, 11:35 PM   #2 (permalink)
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Quote:
Originally Posted by Michaelflat1 View Post
I drive an electronic throttle car, and unfortunately below 1500rpm, if you come off throttle, there will be no fuelcut, if you come completely off throttle, TorquePro shows timing going quite retarded, (maybe -12 degrees) and there is still quite a substantial engine braking effect
Both retarded timing and closed throttle produce engine braking. A closed throttle produces more braking than an open throttle with no fuel.

Quote:
Originally Posted by Michaelflat1 View Post
At the lowest possible throttle position (quite easy to do on an electronic throttle) i can have the same fuel use, and throttle position, but with +21 degrees advance, and a small amount of torque from the engine, an improvement from negative.
More advance (to a point, depending on RPM and other factors) will produce more torque or higher idle RPM or less engine braking. But retard it enough and the engine will brake the same as if fuel were cut. At idle you can get a bit of instant torque just by advancing the timing, which helps overall acceleration (and deceleration) response by controlling timing in addition to throttle.

Quote:
Originally Posted by Michaelflat1 View Post
The fuel use parameter in Torque is not always accurate (it is during warmup), but is skewed with EGR operation, as Torque assumes all airflow is fresh air and does fuel use based on that (i believe)
I don't quite understand what you're saying. Fresh air needs fuel. The engine shouldn't be pouring in more fuel for recirculated exhaust gases. So if it assumes all airflow is fresh then that's a good way to measure fuel, as long as the AFR is the same since the EGR system isn't going to affect how much fuel is added to the fresh air.

Quote:
Originally Posted by Michaelflat1 View Post
So, in these off throttle events, what is going on? Fuel being kept around stoch mixture (14.7:1 AFR) at/near the minimum throttle position? Is the timing purely to reduce torque to a minimum? Or is the mixture going lean and this is because of knock concerns? At roughly 1400rpm off throttle, Torque Pro shows fuel use to be roughly double that of idle, despite the engine producing negative/no torque.

I know that in fuelcut operation (openloop, -15 timing), the EGR and throttlebody are commanded to let some extra air through, and increase manifold air pressure (reducing effective vacuum)
and coming out of fuelcut, the throttle closes and pressure decreases.
Knock is never a problem at closed throttle. You can advance the timing all you want and you won't get knock.

One reason to retard the timing like that is to produce more engine braking while being ready for the engine to start producing torque again (because you hit the throttle again or because it slowed down to idle). That way you already have fuel in the intake and all you need is to bump up timing and you get instant "throttle" response even before the throttle moves.

Another possible reason may be help keep a stoich AFR (for emissions reasons) by opening the throttle a little while not losing engine braking at the same time. I know that this used to be necessary with many carburetors. Maybe a modern car has a hard time figuring how much air is going in at high RPMs and closed throttle.

Or it may do it to keep the engine and catalytic converter warmed up. I've been in older cars that cut fuel and on long downhill descents the engine can cool so bad you lose all heat for keeping the cab heater running not to mention how that might affect emissions.
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Old 12-31-2020, 09:52 AM   #3 (permalink)
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I've found that, in my vehicles, Torque does not take into account air fuel ratio, and it does not measure DFCO at all. I find it completely unreliable for fuel economy figures.

Great post by Isaac Zachary.

I was pretty surprised when I found idle is generally controlled by ignition timing in modern vehicles, and not by throttle, or an idle valve, or fueling. I'm fairly certain this is in large part because those methods would respond too slowly for transient loads (electrical, A/C, letting out the clutch), but it certainly seems wasteful to give way more air and fuel than is necessary to idle the engine, and then keep RPM in place by retarding ignition timing.
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Old 12-31-2020, 11:01 AM   #4 (permalink)
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Quote:
Originally Posted by Isaac Zachary View Post
I don't quite understand what you're saying. Fresh air needs fuel. The engine shouldn't be pouring in more fuel for recirculated exhaust gases. So if it assumes all airflow is fresh then that's a good way to measure fuel, as long as the AFR is the same since the EGR system isn't going to affect how much fuel is added to the fresh air.
Just to clear up this bit, my car is a speed density setup, no MAF, only a MAP sensor.
When EGR is disabled (initially at warmup) the figures match the car mostly

Yet when EGR is enabled, manifold pressure increases, despite similar fresh air 'intake', Torque doesn't know the ratio of fresh air - exhaust gas, and thus assumes more fuel is being injected and Torque's MPG figures drop when warmed up.

If only there was a proper fuel flow PID standard on OBD2

Thanks for the rest of the post, i guess the timing acts as engine braking quite well, high performance VW DSG gearboxes 'fart' as timing is retarded during each gearshift (which happens very quickly) as closing the throttle and adjusting airfuel ratio would be far too slow.
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Old 12-31-2020, 11:59 AM   #5 (permalink)
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Quote:
Originally Posted by Michaelflat1 View Post
no MAF, only a MAP sensor...
Oh ok! I get what you're saying now. EGR would affect manifold air pressure, but so would timing.

On my Avalon when warming up it will retard considerably, like 25 after (not before) TDC. The throttle has to open up much wider and increases manifold pressure (less vacuum) quite a bit. This of course uses a lot more fuel, but that's the point. It does it to heat up the catalytic converter and rest of the engine more quickly. The interesting thing is idle engine speeds can be nearly the same in this more-open-throttle, retarded-timing state as it is when the engine is warmed up and the throttle closes and timing advances back to normal.

When I had my 1972 VW Super Beetle I put on a Magnaspark Digital distributor. I was trying to get it to idle lean since most carbs of that era would idle pretty rich for idle speed purposes. But maintaining a stoich or leaner idle was extremely difficult until I figured out that I could program the distributor to advance as RPMs went down which would help maintain a stable idle speed since the engine would slow down slightly, ignition would advance a little, engine speed would go up slightly, ignition would retard a little, and so on.
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Old 12-31-2020, 01:26 PM   #6 (permalink)
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Quote:
Originally Posted by Isaac Zachary View Post
Oh ok! I get what you're saying now. EGR would affect manifold air pressure, but so would timing.

On my Avalon when warming up it will retard considerably, like 25 after (not before) TDC. The throttle has to open up much wider and increases manifold pressure (less vacuum) quite a bit. This of course uses a lot more fuel, but that's the point. It does it to heat up the catalytic converter and rest of the engine more quickly. The interesting thing is idle engine speeds can be nearly the same in this more-open-throttle, retarded-timing state as it is when the engine is warmed up and the throttle closes and timing advances back to normal.
One thing I've wondered - most Toyota engines use offset cranks, and I'd be shocked if this were not true of the Avalon hybrid's. So, 0 on the crank is not the same as top dead center for the piston. Additionally, I'm unsure if Toyota's "0" is the same as Honda's, or anyone else's. Is it 0 relative to the minimum advance, or 0 relative to TDC, or 0 relative to the crank?
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Old 12-31-2020, 01:44 PM   #7 (permalink)
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Quote:
Originally Posted by Ecky View Post
One thing I've wondered - most Toyota engines use offset cranks, and I'd be shocked if this were not true of the Avalon hybrid's. So, 0 on the crank is not the same as top dead center for the piston. Additionally, I'm unsure if Toyota's "0" is the same as Honda's, or anyone else's. Is it 0 relative to the minimum advance, or 0 relative to TDC, or 0 relative to the crank?
Good question! I was just going by what the Scangauge 2 has to say.

But in the end, the point is that there's no set idle timing on modern cars like old distributor cars. The Avalon apparently will idle anywhere between some 40 difference, depending on which side of the efficient-but-cool/non-efficient-but-warm the timing scale the ECU is shooting for.

Another example is that of old distributor cars with vacuum advance that would change from the ported vacuum advance to straight manifold vacuum when a certain thermostatic vacuum valve reached a certain temperature. This would make the engine advance at idle which would speed up the idle speed and help cool down the hot engine (mainly for emissions reasons).

Also, there have been a few car enthusiasts that would hook their distributors directly to manifold vacuum instead of ported vacuum. The result was that it would idle cooler and use less fuel at idle (assuming the idle speed was adjusted after the change). This also would increase manifold vacuum at idle.

In these two later examples, since both ported vacuum and manifold vacuum are the same anywhere from off idle to full throttle (in 95% of all cars from that time) there's no difference in timing under load on those vehicles.
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Old 01-01-2021, 04:15 AM   #8 (permalink)
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Quote:
Originally Posted by Ecky View Post

I was pretty surprised when I found idle is generally controlled by ignition timing in modern vehicles, and not by throttle, or an idle valve, or fueling. I'm fairly certain this is in large part because those methods would respond too slowly for transient loads (electrical, A/C, letting out the clutch), but it certainly seems wasteful to give way more air and fuel than is necessary to idle the engine, and then keep RPM in place by retarding ignition timing.
That doesn't sound right to me, my Scion FR-S definitely used the throttle to control idle, timing was pretty consistent (until you step on it, then it retards a lot since the engine is knock prone).

When I was messing around with it, if my tune produced a crappier idle, the ECU would automatically run rich to compensate.

What engine did you find that uses timing to change the idle?
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Old 01-01-2021, 09:47 AM   #9 (permalink)
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Best I can tell, all of the Honda engines I've worked on use ignition to fine-control idle. The pre-drive-by-wire vehicles had an idle valve that let in more air when cold, whereas the post-DBW vehicles would simply crack the throttle plate, but both retard ignition significantly at idle.

Take the K24 I'm using from an Acura TSX - it's ~8 years newer but functions the same way as the Insight stock engine in this regard, or our Fit. At idle I'm generally seeing ~5-7 ignition advance. When I flip the headlights on or toggle the A/C it jumps maybe as much as 6-8 additional degrees momentarily to hold the idle perfectly steady, and then levels out at 1-2 more advance to hold the same idle speed. Touch the throttle lightly and it's instantly at ~24 advance, which is ballpark for MBT at that load and RPM.

Here's one of the timing tables my car uses, for example:



Idle timing uses separate logic, and there are different tables for every 10 degrees of intake cam advance, and separate tables for all of these in and out of VTEC.
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Old 01-01-2021, 11:30 PM   #10 (permalink)
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Quote:
Originally Posted by Isaac Zachary View Post
Both retarded timing and closed throttle produce engine braking. A closed throttle produces more braking than an open throttle with no fuel.


More advance (to a point, depending on RPM and other factors) will produce more torque or higher idle RPM or less engine braking. But retard it enough and the engine will brake the same as if fuel were cut. At idle you can get a bit of instant torque just by advancing the timing, which helps overall acceleration (and deceleration) response by controlling timing in addition to throttle.



I don't quite understand what you're saying. Fresh air needs fuel. The engine shouldn't be pouring in more fuel for recirculated exhaust gases. So if it assumes all airflow is fresh then that's a good way to measure fuel, as long as the AFR is the same since the EGR system isn't going to affect how much fuel is added to the fresh air.



Knock is never a problem at closed throttle. You can advance the timing all you want and you won't get knock.

One reason to retard the timing like that is to produce more engine braking while being ready for the engine to start producing torque again (because you hit the throttle again or because it slowed down to idle). That way you already have fuel in the intake and all you need is to bump up timing and you get instant "throttle" response even before the throttle moves.

Another possible reason may be help keep a stoich AFR (for emissions reasons) by opening the throttle a little while not losing engine braking at the same time. I know that this used to be necessary with many carburetors. Maybe a modern car has a hard time figuring how much air is going in at high RPMs and closed throttle.

Or it may do it to keep the engine and catalytic converter warmed up. I've been in older cars that cut fuel and on long downhill descents the engine can cool so bad you lose all heat for keeping the cab heater running not to mention how that might affect emissions.

engine braking is transmission is related..

If i'm in drive no engine braking and the "regenration is not even noticeable " the engine speed is not married


Now on the other hand if I have in in mountain mode or M4 as it's known... Engine braking is noticeable I can come to a complete stop almost .... to about 8mph.. from say 60mph if i go off the throttle.. but I lose CVT 5th equivalent gear, though

as you can see in these videos the v8 engine braking is quite effective..




Last edited by Tahoe_Hybrid; 01-01-2021 at 11:38 PM..
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