HKS Air Fuel Regulator (AFR)

In this feature, Temple of VTEC - Asia's own comments are published in italic fonts.

HKS is one of the premier Honda tuners in Japan. The Malaysian HKS representative is Aerotech and is run by Mr Rennie Khoo, a very knowledgeable engine tuner in Malaysia. Recently, I've had the very good fortune of receiving Rennie's hospitality and we are now embarking on a special feature of tuning products that HKS specially makes for Honda. Drop by often to look out for new products covered. To contact Aerotech in Malaysia, their phone number is 60-3-7556112.

HKS' Air Fuel Regulator (AFR) is a device that modifies the air-fuel ratio of the engine. AFR works by intercepting the air-flow sensor signal (which is a pressure-based MAP sensor in Hondas) and modifying that signal before feeding it to the ECU. This modified signal will in turn cause the ECU to change the air-fuel ratio based on the modified signal. To understand the working principle of the HKS AFR, it then becomes necessary to first understand the working logic of Honda's PGM-Fi fuel injection system.

HONDA PGM-Fi Operating Logic

PGM-Fi controls air-fuel ratio by varying the opening duration of the fuel injectors. The rate of fuel fed by the injectors is constant so a longer or shorter opening duration will increase or decrease the amount of fuel being fed into the cylinders (respectively). There have been a lot of misconceptions and misinformation on the working logic which PGM-Fi uses to determines this injector opening cycle and TOV-A is very grateful to Rennie for the following description.

On the right is a block diagram of PGM-Fi's operating logic for determining injector opening duration. The basic algorithm is : first determine the base opening duration from a fuel map, then compensate for a set of engine and atmospheric conditions which are measured by various onboard sensors. The base PGM-Fi fuel map is a three axes, X-Y-Z type map. The two independent axes are readings from the TDC and MAP sensor, ie engine RPM and air-flow. From these two parameters, PGM-Fi reads a base injector opening duration value off the map. This base value is then modified using the various sensor readings. Some of the sensors are used only during certain situations, eg during engine cranking. There are also different priorities assigned to each parameters which again varies depending on situations.

An important point to note is that although PGM-Fi measures a lot of conditions, it does not measure all atmospheric conditions. For example there is no air humidity sensor.

Headroom, Open-Loop and Closed-Loop Operations

A standard PGM-Fi program is used for each engine design, eg JDM B16A. The base fuel map is programmed using a set of 'base' values for various parameters like air temperature, pressure, and other various other measured engine/atmospheric conditions. From these base conditions, actual amount of oxygen available for combustion will vary according to air-flow as oxygen content per volume of air is assumed to be constant. However, the actual oxygen content available per volume of air will still vary slightly because of other unmeasured atmospheric conditions like humidity, elevation above sea level or even locality, eg countryside or city-center. Because of this, Honda engineers used results averaged from a large sample of varying conditions to create their base fuel map. A similar process was also used to determine the trimming values used when compensating the base injector opening duration for deviations of sensor readings like air temperature and pressure from the base values.

One important design point of PGM-Fi is then the use of 'headroom' (or safety margin) by Honda engineers. To ensure engine reliability, PGM-Fi allows for worst case deviations from the average conditions used in the program. Over-rich air-fuel ratios will cause fuel economy or engine efficiency problems while over-lean ratios may cause detonation and consequently engine damage. So PGM-Fi is designed to run slightly rich for all deviations from the base average conditions used, including the worst case conditions. As a result, the base fuel map, and then the compensations for the trimming parameters are all designed to deliver rich air-fuel ratios under all foreseeable operating conditions.

PGM-Fi operating logic also needs an explanation of what is called the 'open-loop' and 'closed-loop' operating mode for PGM-Fi. Closed-loop means a feedback-loop mode. PGM-Fi works in closed-loop mainly during engine idling. In closed-loop mode, PGM-Fi uses the signal from one critical sensor, the O2 (or sometimes known as lambda) sensor to attempt to attain an ideal air-fuel ratio of 14.7:1 (stoichometric ratio). An O2 sensor operates by measuring the amount of oxygen left in the exhaust gas. The amount of oxygen is a good indicator of the combustion efficiency and a higher oxygen content will suggest a lean condition and vice versa. O2 sensors are normally mounted on the exhaust manifold (or sometimes called the extractors) and/or catalytic converters (for cars equipped with them) and PGM-Fi uses the O2 sensor reading to bump air-fuel ratio up or down until it gets a reading indicating optimum combustion.

So during engine idling, the air-fuel ratio may initially fluctuate up and down based on O2 sensor readings but should eventually settle to a stable value corresponding to the ideal 14.7:1 ratio. PGM-Fi works in closed-loop mode only during idle or very light accelerator openings.

From moderate to maximum throttle openings, PGM-Fi goes into an open-loop mode. In open loop mode, PGM-Fi ignores the O2 sensor signal but will still use the other sensor signals (air temperature and pressure, TPS etc) to make compensations on the base injector opening values. However for very heavy accelerator positions (indicated by large TPS values), PGM-Fi now gives priority to engine RPM, MAP and TPS sensor readings. RPM and MAP sensors will always be required since they are input parameters for reading the fuel-map. But in open-loop mode, PGM-Fi no longer makes as much compensation for other sensor readings, other than TPS sensor, as in closed-loop mode.

When operating in open-loop then, PGM-Fi will usually be operating in a rich-mode. This will be especially true for JDM vehicles when imported directly from Japan into various countries (whether as used or new cars). The PGM-Fi program will have been originally designed based on the atmospheric conditions in Japan and they will almost certainly be different from the countries into which the cars are imported. Even for truly localized line-ups, a rich condition also normally prevail because the PGM-Fi program would have been originally developed to accommodate for the whole country or sometimes even an entire region and thus would again be based on a large base of 'average' values.

Working Principle of HKS AFR

HKS' AFR intercepts the MAP sensor signal to the ECU and modifies it. AFR will either increase or decrease the MAP signal strength by a single fixed quantum value depending on the setting of a dial. This dial has 12 positions (also called 'clicks'). Position 6 is 'neutral' and passes the MAP signal unmodified. The lower 5 positions reduces the MAP signal and the remaining higher 6 positions increases the MAP signal. AFR intercepts only the MAP signal and thus will adjust the MAP signal for all RPMs.

By trimming the MAP sensor value (up or down), AFR causes PGM-Fi to read higher or lower air-flow and correspondingly more or less oxygen for combustion. This will cause PGM-Fi to adjust the injector opening duration thus indirectly changing the air-fuel ratio.

AFR works by exploiting the headroom used in the PGM-Fi program. It is possible to reduce the headroom because we will be using our car in a geographically restricted location and thus more stable atmospheric conditions than those used to design the original program. Thus we use AFR to calibrate our air-fuel ratio to a more exact value to match our locality and this value will usually be different from the base averaged value programmed into PGM-Fi. In addition the deviations from this value will also not be as wide as what the PGM-Fi headroom is designed to accommodate for because of the more stable atmospheric conditions.

Correctly setting the AFR will effectively calibrate the MAP signal to the local atmospheric conditions and this allows extraction of some untapped power from the engine by allowing the ECU to use a more exact air-fuel ratio.

Since we typically will not know the exact conditions under which the original PGM-Fi program is developed, the standard procedure to determine the best AFR setting is through a dyno session. A standard AFR tuning process will involve bumping the AFR adjustment click by one click on either side of the neutral click 6 setting and dynoing the resulting power gain/loss. The final setting will be that where maximum power is obtained without any adverse side-effects like knocking etc.

Rennie goes to great lengths to explain that the AFR does not directly modify the amount of fuel fed into the engine, ie AFR does not touch the injector output signals from the ECU and the ECU still has total control over that signal. AFR modifies the MAP signal and that influences the ECU to richen or lean the air-fuel ratio by detecting higher or lower amount of air being fed into the engine. This is a very different implementation. The ECU still has throttle position, engine RPM and the other minor signals (engine temperature, O2 sensor output, etc) to consider together with the modified MAP signal to determine the injector opening duration. This is not exactly the same as modifying the actual injector opening duration control signal.

One common criticism of the AFR is that it has only one adjustment point for the entire RPM range. Compared to competing products with multiple rpm point adjustments, it is suggested that AFR is limited in its ability to tune engine power. This criticism is invalid. AFR is designed to work with stock or mildly modified engines. Mild modifications means the normal filter-exhaust type changes, ie those which improves air-flow through the engine. Notice that AFR is effectively recalibrating the air-fuel ratio to local atmospheric conditions and this calibration is constant regardless of engine RPM (ie does not vary on engine RPM). Therefore, AFR needs only a single adjustment point. This is also the reason why it also works for stock engines.

For moderate to heavy mods, eg camshaft changes or headwork, it may now be necessary to adjust the actual air-fuel ratios differently across different parts of the rpm-band. Now, AFR may no longer be as effective and the multi-rpm adjustable devices like HKS GCC-2 or Field SFC-VTEC will be necessary. Nevertheless, HKS maintains that the most accurate adjustment device for such cases will remain those that actually modifies the injector opening duration values, eg HKS FCON-4 or FCON-5 or an ECU change.

The AFR is installed on a brand new 1997 Malaysian Domestic Market Civic EXi, using a 1.6lSOHC PGM-Fi engine and automatic transmission. The engine is spec'ed to give 120ps at 6400rpm. The 1st run was done after doing 1040km on the car (complete running in) and after the first service which comprised engine oil change (Shell Helix), and oil-filter change. This car dyno'ed at 85.2ps for the base run working out at a 29% power loss due to peripherals and the automatic transmission. The after run is with a HKS AFR set at click 5 (one point leaner air-fuel ratio than stock) and with cleaned fuel injectors. The new power is 90.0ps at the wheels or a 4.8ps increase. 90ps at the wheels works back to 126.8ps at the engine, about a 5% increase. The fuel injector cleaning gave slightly less than 1ps increase in max power.

The AFR is installed on a Honda City 1.3EX manual. The City is a special regional model for the South-East Asia market, conceptualized as a special "people's car" by Honda. The base engine is spec'ed to produce 90ps at 6300rpm. The before run was on a totally stock engine and the after run was with a HKS AFR set at click 4 (two points leaner than stock) and also with cleaned fuel injectors. With no other changes, the final power from the combination of AFR and cleaned injectors was 83.2ps from 76.9ps at the wheels or a 6.3ps increase. 83.2ps works back to almost 98ps at the engine, or a 9% power increase over stock !

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