Analysis of Thermal Cycles ME302

Analysis of Thermal Cycles ME302


Laboratory Experiment

Spark Ignition Engine : Effects of Variation of AFR and Spark Timing


To investigate the effectsof Air to Fuel Ratio and Spark Timing on engine performance. The engine performance is directly measured by dynamometer as brake torque. The measurements from the Engine Control Unit are also used to determine volumetric efficiency and brake specific fuel consumption.


Ford CVH 1.4 litre, 4 cylinder, port fuel injected engine,

Plint electrical dynamometer,

Programmable ECU, Reata.

Data logged by ECU :

from sensors on engine : Mass Air Flow g/s, Manifold Absolute Pressure kPa, Air Temp oC (in column Air), AFR from wide band O2 sensor, Brake Torque (lb-ft)

other: RPM, Duration Of Injection per injector per 2 crank revolutions milli-seconds (in Column Fuel), Spark Advance crank angle.


  1. Engine is started and warmed up to operational temperature, i.e. with opened thermostat 70 or 80 Celsius.
  2. The engine is loaded to a particular RPM and Load as directed during the laboratory session. The fuel quantity is then varied by changing Duration Of Injection to generate an AFR sweep from around 11 to 17(AFR measured by wideband O2 sensor in exhaust). The torque should be noted to vary with AFR. Note that all data being reported on screen is logged by the computer and will be available for analysis after lab session. A DOI that generates and AFR of around 14 is then recorded into the ECU.
  1. At the same RPM and Load at an AFR close to 14, the spark advance is varied from say 5 degrees Before Top Dead Centre to say 50 degrees BTDC (but limiting to say 40 degrees BTDC if close to Wide Open Throttle). The toque should be noted to vary with spark timing.
  1. Repeat the variation of AFR and Spark Timing as above for two other set-points of RPM and Load.


The data logs can be opened and manipulated in Excel. Note that the logs might have portions of data that are from the ramping up or down to and from the actual set point of RPM and load. Also data might have a repetition of say AFR and torque for example due to the fact that AFR is made to vary from 11 to 17 and then back to 14 generates two portions of data with AFR 14 to 17.

For each set point of RPM and Load,

1: Plot Torque (Nm) versus AFR, AFR as logged in the computer data logs whichis as measured by the wideband O2 sensor.

2: Compute AFR from logged DOI and measured Mass Air Flow . The DOI can be used to calculate the fuel quantity injected per cylinder every 2 revolutions by noting the fact that the injectors in this engine flow around 0.003 ml/ms. Compare the computed AFR to the AFR measured by the wide band O2 sensor.

3: Plot Specific Fuel Consumption (g/kWh) against AFR.

4: Calculate volumetric efficiency based on measured and logged MAF. Also calculate volumetric efficiency based on calculated mass induced into cylinder if the cylinder is filled to MAP pressure at BDC at the measured air temperature ( or say if air was heated up to coolant temperature).

4: Plot Torque versus Spark Timing.

5: Plot BSFC versus Spark Timing.


Draw your own conclusions on the experiment and results obtained.

The Lab Report is to be handed back in printed format. Note that the number of curves being plotted is not large and hence colour is not really required. It is better to just plot dta point without any connecting lines so that the quantity of data is visible and excess clatter by lines is avoided.


Mario Farrugia October 2012