Engine Calibration

What are the primary steps to make a base engine calibration and how are all the numbers calculated?

Introduction

Engine Calibration involves setting configurable parameters stored in the RAM (Random Access Memory) of an engine controller, in order to tune the strategies to run the engine in the best possible way in all its operating conditions. This article starts from the basics and uses as and example the 8.4L V10 from the rather rapid Dodge Viper ACR.

Dodge Viper 8.4L V10 Engine

Bore: 103.0 mm

Stroke: 100.6 mm

Con-rod length: 158.55 mm

Compr ratio: 9.6:1

Intake Valve dia: 52.8

Exh Valve dia: 40.89

Intake Valve lift: 14.4 mm

Exh Valve lift: 13.59

Intake Valve open: 41deg BTDC

Exh Valve open: 129deg BBDC

Intake Valve close: 123deg ABDC

Exh Valve close: 41deg ATDC

Air Mass Flow

In order to understand how much fuel must be injected, it is necessary to understand how what mass air flow the engine can "ingest". For the Viper V10 we are fortunate that some mathematical models have calculated the amount of kgs per second for various engine speeds. In real life the flow is measured by a hot wire anemometer where flow rate is proportional to air velocity through a fixed cross sectional duct. The wire uses a "seeback effect".

Q = A * v

At the lower engine speeds, for our example "large" engine we have the following values:

1500 rpm: 0.07 kg/s

2000 rpm: 0.11 kg/s

2500 rpm: 0.15 kg/s

3000 rpm: 0.18 kg/s

At the maximum power point of 6000 rpm we can see that the plot value of 0.45 kg/s closely matches the 0.465 kg/s value that we can determine by hand calculation:

Fuel Mass Flow

Once the air flow rate has been calculated, it is possible to estimate the amount of fuel to be added, for combustion. I quite frequently double check my answers with those from an established motorcycle engine builder, such as RB Racing in the USA. It uses imperial units rather than metric but the conversions are quite simple.

Then we have to calculate the time that an injector can be open for at any given engine speed, for example the maximum engine speed. In the case of 6000rpm, the time is calculated like this:

6000 rpm = 100 rps

since there are two engine revolutions to complete 4 strokes: 2 / 100 = 0.02 s (20ms)

An injector cannot work effectively when it is switched ON all of the time. It works best at low to medium duty cycles, with a common maximum of 85%. So at 6000 rpm, the injector would not work well if it stayed open longer than 17 ms.

How much fuel can be delivered by the injector depends on its flow rate. So for our example engine, the standard injectors are 38 lb/hr (400 cc/min circa) when running at 43.5 psi (3 Bar). So for our engine we can calculate that if the injector can do 400 cc in one minute, then it can do 6.667 cc in one second. And at the speed of 6000 rpm it can flow 0.113 cc.

Air Fuel Ratio

A perfect mixture of air and gasoline fuel happens with a mass ratio of 14.7:1 (the largest component being the air). However to produce highest performance and cool the combustion, the ratio is typically lower and this requires more fuel. Values of 11.1:1 are common.

The quality of the mixture (Lambda) is measured by the Lambda Sensor, which actually measures the free oxygen in the exhaust gas.

For our example engine, each cylinder is 840 cc in size. So "in theory", if it is to be homogeneously filled for good combustion, it would require 57 cc of fuel. [needs validation]

Fuel & Ignition map

The Engine Control Unit will control the injectors based on a fuel map that defines the aperture time of the injections. It will also control the coils with a map that defines the exact spark timing.

* Ignition advance reduces as load increases: cylinder filling is better and the mixture burns faster

The examples below are from a good website called enginetuner.com.

For what is called a "Base Fuel" map, we consider not only the engine operating speed but also the load, which is often measured with a MAP (Manifold Air Pressure) sensor. This responds very fast to transient conditions, compared to a MAF (Manifold Air Flow) sensor.

This Base Fuel map was calculated with a freeware Excel tool: Injection Time Calculator

Injector & Spark Plug Hardware considerations

It is important to realise that any hardware has some limitations. For the case of an injector, it has an internal moving mass and related delay in opening. This is typically called "dead time" and is between 1.0 and 1.6ms, with 1.3ms a good average starting value. The value can change with battery voltage so the ECU will have correction tables of time vs voltage.

Also, it is important to note that for example an injector dead time might be 1.3ms at 20C but 1.2ms at 90C, so temperature is also a factor to remember when running a calibration session on dynamometer.

Modifiers for Fuel and Ignition

It is important for the engine control unit to monitor the "operating environment" and conditions of the engine

Cranking Fuel: when the engine is being cranked at low speeds of 200 rpm approx, the port air speed is low and not very good to atomise the fuel. So the map will have a percentage addition of fuel when cranking, also with a relationship with temperature.

Cold ambient Fueling: when the engine is below operating temperature, the fuel will not atomise fully, combustion chamber temperature will be low and this will lead to incomplete and slow combustion. Hence the engine will require additional fuel. The map will contain a percentage of extra fuel based on coolant and/or oil temperature. When air temperature is high, its density decreases and the oxygen available for combustion also. Then the mixture will become too rich. The maps will contain a percentage reduction in fuel based on intake air temperature.

Knock sensors listen for pre-ignition and can adjust the timing.

Acceleration Fueling: when the throttle is opened quickly then the mixture may become lean as air is drawn in faster than the fuel. The fuel may already have gone through the inlet valve before the change in inlet flow/pressure has time to happen. The weaker mixture may lead the engine to stutter and hence the map will have settings to add and extra percentage of fuel based on throttle position. This fuel is then decayed after some time.

Deceleration Fueling: the map can decrease fuel quite rapidly if the throttle is closed suddenly in order to reduce unburnt fuel and hydrocarbons.

Traction Control

Traction Control is a feature to limit torque in order to keep the tyre contact patch safely on the road. An engine management system can alter the fuel and spark or the throttle angle if the vehicle is fitted with drive-by-wire.

Volumetric efficiency estimation

In order to create a Base Fuel Map, it is important to have a better idea of the engine volumetric efficiency. This can be approximated by this quadratic equation:

VE = A*rpm2 + B*rpm + C

APPENDIX: Engine Data

Volumetric Efficiency Table

EVEME 1.0 //VEX format version

UserRev: 1.02

UserComment: Automatically generated

Date: 05-14-117

Time: 14:44

Page 0

VE Table RPM Range [ 8] # expressed in RPM/100

[ 0] = 7

[ 1] = 12

[ 2] = 21

[ 3] = 30

[ 4] = 39

[ 5] = 47

[ 6] = 56

[ 7] = 65

VE Table Load Range (MAP) [ 8]

[ 0] = 20

[ 1] = 30

[ 2] = 45

[ 3] = 55

[ 4] = 65

[ 5] = 80

[ 6] = 90

[ 7] = 100

VE Table [ 8][ 8]

[ 0] [ 1] [ 2] [ 3] [ 4] [ 5] [ 6] [ 7]

[ 0] = 23 23 23 41 54 59 58 55

[ 1] = 23 23 23 44 58 63 62 59

[ 2] = 23 23 24 48 63 69 68 65

[ 3] = 23 23 25 51 67 74 72 68

[ 4] = 23 23 27 54 71 78 76 72

[ 5] = 23 23 29 58 77 84 82 78

[ 6] = 23 23 30 61 80 88 86 82

[ 7] = 23 23 32 64 84 92 90 86

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