Ethanol & Flex Fuel Tuning: Ignition Timing
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Ignition Timing
07.50
| 00:00 | - One of the more significant areas you may want to address when tuning on ethanol based fuels is the amount of ignition advance applied by the ECU. |
| 00:09 | This is one of the main areas that ethanol gives us the potential for a power advantage over gasoline. |
| 00:15 | Particularly on turbo charged engines running on gasoline, we'll usually find that the ignition timing is limited by the knock threshold. |
| 00:24 | This means that the engine begins knocking or detonating before we reach peak torque. |
| 00:30 | Before we go too much further, let's revisit what we're trying to achieve when we're optimising the ignition timing. |
| 00:37 | Remember that what we are trying to achieve with our ignition timing is to start the ignition event at the correct point in the engine's cycle so that we achieve peak cylinder pressure somewhere around 16 to 18 degrees after top dead centre. |
| 00:53 | This is where we can achieve maximum mechanical advantage from the cylinder pressure acting on the top of the piston, and hence convert this into torque at the crankshaft. |
| 01:04 | This point is known as minimum timing for best torque, or also known as maximum brake torque timing, and is referred to as MBT. |
| 01:14 | If you're tuning a turbo charged engine on pump gasoline, in my experience around 95% of these engines will be knock limited. |
| 01:23 | This means that to prevent the onset of detonation, we've had to purposefully retard the ignition timing from MBT, or more likely we never achieved MBT timing in the first place. |
| 01:36 | This adversely affects the peak cylinder pressure achieved as well as where in the engine cycle we see this pressure peak. |
| 01:44 | Ultimately this affects the amount of torque and power that the engine produces. |
| 01:50 | The other downside is that as we retard the ignition timing the exhaust gas temperature increases. |
| 01:57 | As we've already discussed, the higher octane rating of ethanol coupled with the cooling affect it has on the combustion charge will make the engine much less likely to suffer from detonation. |
| 02:09 | This in turn allows us to add timing back into the regions where we were previously knock limited, and often we can tune the ignition timing to achieve MBT. |
| 02:20 | I'll cover a common myth here that an engine won't or can't knock on E85. |
| 02:26 | While the chances of knock are greatly reduced on E85, it's incorrect to assume that we're now immune to knock, and it's still critical to monitor for knock while we're tuning. |
| 02:39 | If we're starting with an ignition map that's been optimised for gasoline, it's not uncommon to add 10 degrees or more timing to the on boost areas of this timing map on a turbo charged engine. |
| 02:51 | It's important however to understand there is no definitive rule for how much timing we can or should add. |
| 02:59 | This will depend mainly on how severely knock limited the engine was on gasoline. |
| 03:05 | The other aspect that's often overlooked is that if the engine wasn't previously knock limited on gasoline, we're likely to find that the engine actually wants less timing to reach MBT on ethanol blended fuels. |
| 03:20 | This may seem counterintuitive and it certainly goes against what most tuners would consider for E85. |
| 03:27 | But it's the result of the way ethanol burns. |
| 03:31 | E85 actually has a faster combustion speed than gasoline. |
| 03:36 | When we're selecting the ignition advance what we're doing is trying to select the right point in the engine's cycle to ignite the fuel and air mixture in order to achieve that peak cylinder pressure at the correct point. |
| 03:49 | All else being equal if the fuel burns faster we want to start the ignition event later so that we still arrive at peak cylinder pressure at the same point. |
| 04:00 | In my experience the differences between an optimally tuned ignition map between gasoline and E85 in an engine that isn't knock limited, is normally minimal, perhaps in the range of a few degrees, so it's not a large difference but it is still a factor we need to understand and consider. |
| 04:20 | The best way to deal with optimising the ignition timing on an ethanol based fuel, is to simply treat it in the same way as we would tune the engine on any other fuel and slowly advance the timing on a load bearing dyno while monitoring the effect on torque and listening for knock. |
| 04:38 | We can continue advancing the timing until we either reach MBT or alternatively the engine becomes knock limited again. |
| 04:46 | So essentially what we can expect to find is that the amount of additional ignition timing that the engine will need compared to gasoline will be dependent on how heavily knock limited the engine was. |
| 04:58 | We'll usually find that in the light load areas of the map such as cruise where the engine wasn't previously knock limited, that the engine may actually want less ignition timing. |
| 05:09 | Likewise as we increase the load we'll find that we may need to add proportionally more timing compared to what we had on gasoline. |
| 05:17 | Regardless of the load or RPM we're tuning at the dyno will show you what the correct ignition advance is. |
| 05:25 | If you're tuning a flex fuel system you'll also need to define how the timing will change with regard to ethanol content. |
| 05:33 | In this situation what we have is an ignition map for gasoline and either an ignition map or a compensation map for ethanol fuel. |
| 05:42 | And then a blend table which defines how much of the ethanol timing map or compensation map the ECU will use as the ethanol content changes. |
| 05:53 | We're going to discuss this concept in more detail in a separate module though so don't worry too much about it now. |
| 06:01 | If your'e tuning at a variety of ethanol contents between E0 and E100 we also need to understand that the effect of ethanol content on the engine's knock resistance is not linear. |
| 06:14 | What I mean by this is that we tend to see a very sharp increase in knock resistance as we move from E0 or pure pump gasoline through to around E35 to E40. |
| 06:27 | As we move from E40 through to about E70 to E80, we still see an improvement in knock resistance but it's much less than what we saw initially. |
| 06:37 | Beyond E80 there's typically little advantage to be had from those higher ethanol contents. |
| 06:44 | So what we've found in this module is that if the engine isn't knock limited on gasoline, the actual ignition advance that will provide optimal torque is often similar or even lower on ethanol than on gasoline. |
| 06:58 | This is important because many tuners falsely assume that since they've switched from gasoline to E85, they'll need to add 10 degrees timing everywhere before they get started with their tuning. |
| 07:11 | We also need to understand that the more heavily knock limited the engine was on gasoline, the more timing we'll likely need to add in those areas when we move to ethanol. |
| 07:22 | Remember also that the relationship between knock resistance and ethanol content is not a linear one. |
| 07:30 | With ethanol's ability to suppress knock, it's very easy to over advance the ignition timing without encountering detonation or knock. |
| 07:38 | While the engine may not knock in this condition, the over advanced timing will still be counterproductive and reduce the amount of power and torque the engine makes. |
