Engine RPM is a straightforward metric however engine load may be a variety of inputs. i.e. Engine load may be throttle position and even though this again is straight forward however there are occasions when throttle position is not the optimum input to use as a measure of engine load. For example, there may be instances when the throttle position is fixed at say 50% throttle and 3,000 RPM but turbocharger boost pressure may be anywhere from vacuum through to full boost. This discrepancy may require additional inputs such as manifold pressure (MAP) or mass air flow (MAF) to be used to better quantify engine load. Indeed, there are occasions when one engine load strategy is appropriate in a particular part of the engine’s RPM range, but then switch to another load strategy in another RPM range. The scope of this article however is not to discuss the merits of each engine load strategy, but moreover to explain the mapping process and equipment required.
Mapping an engine management system involves inserting appropriate values into each of the data points of the fuel, ignition timing and boost tables shown above. This may sound uncomplicated however discovering the optimum value for each data point is a demanding process and one that borrows heavily upon the skills of the tuner and the equipment he has at his disposal.
Let us now run through a number of tuning steps to have a better idea of the mapping process. The steps listed below are a subset of the total tuning task (for the sake of simplicity), but they will highlight a number of areas where different factors influence the tuner’s strategy in achieving the ultimate goals.
1. Engine idle
This step involves inputting data for one data point at zero engine load and idle engine speed. For this point, the tuner monitors the engine’s air-fuel ratio (AFR) and adjusts the fuel deliver to achieve the target idle AFR. Similarly, ignition timing is set to achieve smooth engine idle.
This process is indeed uncomplicated and easy to set. It should be noted at this point that most modern engines run in what is known as “closed loop” operation at this point. This means that the AFR is constantly monitored by the engine management system (with feedback from the oxygen sensors in the exhaust system) to constantly trim the fuel delivery in order to consistently achieve the target AFR.
2. 1,000 RPM and 10% engine load
This area of the mapping process is surprisingly important in delivering a car that “feels good” in normal day to day driving, particularly in stop-start traffic. This area of the map however is one of the more difficult areas to tune for and one that is often neglected.
As opposed to the previous step where the engine is unloaded, the tuner must now have the ability to hold the engine under load and keep the engine RPM fixed at a constant RPM. This is done using a load based dynamometer. In addition, because the engine is most likely little power and torque when compared to the maximum power and torque the result of any tuning changes must be measured with a very accurate dynamometer.
Tuning again involves achieving the optimum AFR and ignition timing. By altering each variable and monitoring the results on the dynamometer, the optimum values can now be entered into the mapping tables. Whilst the engine may be producing only 5 hp at this point, this may net 10% to 20% improved power, the resultant improvement in throttle response can be dramatic and well worth the effort.
3. 1,000 RPM and 100% engine load.
This is another area that is very important in day to day driving and one that typically requires vastly different AFR and ignition timing values than the previous example.
The engine must again be placed under load and the RPM held constant at the required speed (whilst at full throttle/full load). The tuner monitors the results of his tuning changes and enters the optimum values into the appropriate mapping tables.
Like the previous example, this type of mapping requires a load based dynamometer that can hold the RPM constant regardless of the load. An inertia based dynamometer is clearly an inappropriate tool to use because of its inherent inability to hold load at constant speed. In addition, road tuning is inappropriate because it is impossible to measure the result of any tuning changes and whether or not a tuning change has had a positive or negative result on engine output.
4. 2,500 RPM and 10% engine load.
This area of the tuning map is often where much of the driving is conducted - in other words, cruising with the traffic. This is an area where not the maximum engine efficiency is desired in order to achieve the best possible fuel economy.
Again, the engine must be held at a constant speed under load on a load based dynamometer and the optimum AFR and ignition timing is achieved. In these cases, it is not uncommon to enlist the help of additional monitoring equipment such as a 5-gas analyser to monitor the exhaust gas constitution as tuning changes are performed. The gas analyser results give a good indication of the combustion process so as to assist the tuner to maximize the engine efficiency in this part of the engine’s operating range.
This also happens to be the area where many turbocharged vehicles generate significant positive manifold pressure (boost) and boost mapping may be required in addition to AFR and ignition timing.
5. 2,500 RPM and 50% engine load.
This is an area of the engine management map that is used when for example one is driving up an incline. Again, in steady state (engine RPM constant) a load based dynamometer is required to simulate these conditions and provide visual feedback of the results of changing the tuning variables. The tuning process is much the same as that described previously.
At this point however it may be wise to introduce another aspect of tuning transient response. This involves dynamically driving through the target engine RPM (at 50% engine load) in order to make adjustments to the tuning maps if necessary. This is one aspect of simulation that inertia based dynamometers can perform however because there is no ability to alter the dynamometer inertia (or rate of acceleration), their functionality is limited. Load based dynamometers however do have the ability to alter the acceleration rate in order to simulate rapid changes in engine RPM (such as accelerating in 1st gear) or slow changes (accelerating in 6th gear).
6. 2,500 RPM and 100% engine load.
In this part of the map, the engine will no doubt be delivering strong torque, particularly if it is turbocharged. This is also an area where the tuner would likely begin to pay particular attention to the ignition timing in order to avoid detonation. Again, it is particularly important to hold the engine under load for a short period of time so that the engine reaches a steady state of operation at the desired load/RPM site. When the engine is at steady state, the tuner can then confidently input the optimum tuning parameters into the fuel, timing and boost maps to ensure the most powerful and safest engine operation at this point. Without a load based dynamometer, this is impossible to achieve.
7. 4,000 RPM and 100% engine load
Many modern turbocharged engines deliver their peak torque figures in this operating range and coincidentally this is also where they experience peak cylinder pressures. Tuning for engine safety in this range is vital and it is not uncommon for the tuner to begin reducing turbocharger boost pressure , retarding ignition timing and increasing the fuel delivery (richer AFR in order to slow the flame front during combustion) for reasons of engine safety. It is also an area where the tuner must also consider the mechanical loads on the engine (and indeed the rest of the drive train) in addition to the engine operation parameters.
Holding the engine under load allows the tuner to fully assess the total load on the drive train, but more importantly, the tuner has a period of time to listen carefully to the engine whilst operating the vehicle on the dynamometer (with advanced acoustic equipment). The tuner can hear the onset of any detonation before any engine damage or other computer intervention occurs (via knock sensors). This task is very difficult (arguably impossible) to perform without the use of a load based dynamometer capable of holding the engine at constant RPM whilst under full load.
8. Maximum RPM and 100% engine load
Contrary to popular belief, this is one of the easiest areas to tune for. Whilst it is rare to encounter driving conditions where peak RPM is held for any period of time at 100% engine load, the tuner should cater for this situation by holding the engine at high RPM at 100% load to ensure that the fuel, ignition timing and boost parameters are optimum. This can be easily accomplished on a load based dynamometer however if only an inertia dynamometer is available, it may take many attempts under these severe operating conditions to achieve the desired results.
As we have seen from the above, there are many factors that affect the target tuning parameters during the process of engine mapping. These factors vary depending upon the engine operating conditions (engine load and RPM) in order to implement the optimum state of engine tune and achieve the goals of:
- High Power
- High Torque
- Frugal Fuel Consumption
- High Engine Safety
- Low Exhaust Emissions
- Crisp Throttle Response
A common requirement for all but engine idle mapping is a load based dynamometer. One that will allow the tuner to hold the engine at a set RPM regardless of the engine load - in order to achieve steady state operating conditions and tune accordingly.
In addition, a load based dynamometer gives the tuner the ability to better simulate road conditions (whilst actively viewing all relevant monitoring equipment in real time) by setting the appropriate acceleration rate on the dynamometer for the desired simulation.
The dynamometer however is simply a tool, but only when in the hands of a highly experienced and knowledgeable tuner can the optimum state of engine tune for all combinations of engine RPM and engine load be achieved.