Time Advance Processor for LPG(Propane) 4

[rdd48856]

I would like to know if there is any advantage in using this devices on modern cars(with knock sensors). I found controversial answers to this simple question. LPG Kit installers told me there aren't any advantages but they can't say why. It is very hard to understand their language...they talk about things like "the engine get used to LPG" but they can't explain it in technical language. I know that LPG burns slower than gasoline so you need more spark advance at lower to medium revs but less spark advance at high revs(I don't know the reason for this, thought). However if you have active knock control the ECU will always ignite near detonation(is this right?). If so is this option(detonation near knocking) always the best(thermodinamicly it is but for engine and cat life span, I don't know). Can someone explain me better how knock sensor information is used by tge ECU.
Thank you very much...Happy new year

 

[CESSNA1]

RDD48856: As you said the nowck sensor is used to prevent damage to the engine. It typically picks up the sharp spike (knock) associated with engine knock. It feeds this signal to the ECU which then changes the air/fuel ratio and / or retards the spark.

The disadvantage of taking to installers is that they are just that-installers. They usually know little or nothing about the product they are selling. this lack of technical knowledge results in the vague, techno-babble answers you are receiving. This works for most people who are not technical. Engineers however, drive them nuts. Keep up the good work.

Regards
Dave

 

[OldFredBear]

The ECU takes the knock signal, filters the signal and extracts (if any), the knock signal via FFT. It will then advance/retard the spark appropriately. For LPG you would need to have a specific spark table, not just the stock ecu that is set up for pump gas.

Also the knock system is usually limited to an advance/retard limit, just in case the knock system goes insane it would clip to a safe point, thus saving your pistons/cat converter/and engine overall.

 

[patprimmer]

As I understand it, and it is not my field of expertise, the primary purpose of the knock sensor is to improve performance and economy by allowing the ECU to push the limits re ignition timing for whatever conditions the engine happens to be running in.

Without the knock sensor, a safety margin taking worst case scenario into account must be used, thereby costing power and economy via reduced thermodynamic efficiency

Regards
pat pprimmer@acay.com.au
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[patprimmer]

I forgot to mention that I would expect that the previous comment by knockeng re limits to the range of control via the knock sensor makes a lot of sense to me, so a look up table specifically for the fuel/engine combination would be the required starting point, so as to keep the knock sensors job within reason.

Regards
pat pprimmer@acay.com.au
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[franzh]

In some cases, when using LPG with no ignition support, the knock sensor does not detect a knock and the ECM bumps timing to its maximum level, then drops it to failsafe to prevent any "detonation", as the ECM has no knowledge of which fuel is being used. It may assume that the knock sensor is defective and may set a knock sensor code. The failsafe may be as much as -20 degrees from optimum.
LPG engines can benefit from timing enhancements at low and part range throttle for all the reasons stated above. At full or heavy load, no additional advance is really needed, not especially sure, but dyno test have confirmed it.
Franz

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[rdd48856]

Thank you very much for all the answers you gave me. They are very useful. Now I understand much better how knock sensors and TAPs work. I even e-mailed to several installers and kit manufacturers about TAPs and they all answered me about the same. There’s no need because modern cars have knock sensors and bla, bla,bla. A guy told me that multipoint sequential LPG injection kits are able of delivering the correct amount of fuel needed (which I believe is true) so there’s no need of TAPs (what’s the link between both?). Another told me that active knocking detection does the ECU continuously change the ignition curve. Manufacturers told me that you have to use TAPs for Natural Gas but not for LPG. But they didn’t give me any reason for this…
Thank you again…Any additional information is still welcomed.

 

[rdd48856]

Thank you very much for all the answers you gave me. They are very useful. Now I understand much better how knock sensors and TAPs work. I even e-mailed to several installers and kit manufacturers about TAPs and they all answered me about the same. There’s no need because modern cars have knock sensors and bla, bla,bla. A guy told me that multipoint sequential LPG injection kits are able of delivering the correct amount of fuel needed (which I believe is true) so there’s no need of TAPs (what’s the link between both?). Another told me that active knocking detection makes the ECU continuously change the ignition curve. Manufacturers told me that you have to use TAPs for Natural Gas but not for LPG. But they didn’t give me any reason for this…
Thank you again…Any additional information is still welcomed.

 

[franzh]

Many installers do not add additional timing processors for LPG since the octane is relatively close to gasoline but do for CNG since that octane is around 125.
Personally I like to include a timing processor or remap a conventional distributor for whichever fuel I am using, it does indeed help squeeze performance. I would guess up to about a 10% horsepower improvement with a properly timed engine, on LPG compared to an LPG engine without additional timing. Also, most modern sequential injection systems do indeed connect to the ignition primary and will modify the timing curve, although a seperate device is not installed.
Note that the key here is that the system is a complete package, not one of the older piece-by-piece systems.
Franz

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[1Bluegrass]

I am Ford savvy and here is the deal for the way the Knock sensor is used.
A check of all the EFI truck and car models since 1985 to about 94 for 5L and 5.7L engines shows this sensor is used only on the light trucks.
Why; because it is a protection for the engine from low rpm heavy loading causing engine damage from PING type of ignition that is too advanced for the load and octane of the fuel being used.
A secondary result of using a knock sensor is that ignition timing can be sort of crowded upwards (advanced) allowing the engine to make more low rpm torque in an unloaded condition plus a bit extra compression ratio.
The knock sensor is a quartz element that produces a voltage output that is processed by the ECM and applied to the spark advance tables causing a retard to occurr. The retard range can be as high as 8 degrees.
Once the knock program has been entered, the return to regular timing may not occurr instantly.
Going futher, high rpm knock is another story brought on by other conditions. Excess temperature, fuel leaness, lack of lubrication in the cylinders, excess oil burning etc.
Many heavy cars use the knock sensor with the smaller engine for protection from over load at low rpm such as the 4.6L mod engine in Lincolns.
Also keep Ping, pre ignition and Detonation seperated because they can be caused by differing circumstance.
As to the difference in design/operation when using an alternate fuel, i'm sure there are preferred operating parmeter such as might be found in testing and programmed into the ECM program after the alternate fuel detector has found the situation.
Some Ford vechicles are already alternate fuel equipped and their owners don't know it unless they read their owners manuel.

 

[rdd48856]

Thank you again. Franzh, that´s what I first thougth when they told me they didn´t install TAPs anymore. But I asked them and looked to the install schemes and there wasn´t any direct connection to the ignition. The systems have their own ECU connected to the petrol ECU via OBD. So maybe LPG ECU is capable od sending information to the gasoline ECU. But when I asked them about this they answered me that no information is being send to the gasoline ECU. I must stress that lpg on south of Europe is mostly butane which has an octane rating near gasoline(but still different flame speed...

 

[franzh]

There are several good conversion systems that do communicate with the OEM PCM via the communication bus OBD link. Diagnostics, MIL light, adaptive learning, injector pulse, knock signals, etc are all translated this way.
Franz

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[rdd48856]

But is it possible to send(not only receive) information capable of changing ignition time in real time, that is when fuel is turn to LPG, to the gasoline ECU via OBD?
Thank you again.

 

[franzh]

I guess that would depend on the designer of the conversion package. Technically, it is possible.
Franz

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