Eastern Mediterranean University
Department of Mechanical Engineering
Laboratory Handout
COURSE: MENG422 AUTOMOTIVE ENGINEERING I
Semester: Fall (2010-2011)
Name of Experiment: Consumption loops of a S.I. engine
Instructor: Assist. Prof. Dr. Hasan Hacisevki
Submitted by:
Student No:
Group No:
Date of experiment:
Date of submission:
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EVALUATION
Activity During Experiment & Procedure
30 %
Data , Results & Graphs
35 %
Discussion, Conclusion & Answer to Questions
30 %
Neat and tidy report writing
5%
Overall Mark
Name of evaluator:
OBJECT: To obtain the effect of mixture strength on the performance of a S.I. engine.
APPARATUS: TD 43 Variable compression engine test rig complete with
necessary instrumentation.
THEORY: A spark ignition engine is capable of operating on a range of air/fuel ratios (mixture strengths), by weight,
from about 8:1 to 22:1. The former corresponds to a rich mixture (excess fuel) and the latter to a weal mixture (excess air).
The general performance of the engine in terms of power output, economy, production of pollutants in the exhaust and
general smoothness of operation depends critically upon the mixture strength. It is the function of the carburettor to
regulate the mixture strength to the most suitable value over the whole range of engine operating conditions. These involve
the full speed range, the full range of load, accelerating, overrun and idling conditions. These requirements, make very
exacting demands upon the carburettor and induction system and explain reason for the complicated nature of modern
carburettor design.
An understanding of this important matter may be achieved relatively simply by the use of a carburettor fitted with a
variable “Jet” . This consists merely of a needle valve associated with the main fuel supply jet of the carburettor by
adjustment of which the amount of fuel admitted to the air stream may be varied and the mixture strength altered at will.
The experimental procedure is to run the engine at constant speed and full throttle, to vary the mixture strength
progressively and to take a series of measurements of power output and fuel consumption. Difficulty may be experienced
in running the engine both when the mixture is too weak or too rich. The former leads to “popping back” in the airbox and
places a limit on the lowest setting of the mixture screw, whilst too rich a mixture leads to “rich extinction”. This places a
limit on the maximum setting of the mixture screw.
When the test results are presented in the form of specific fuel consumption against brake power the resulting curve is
called the consumption loop. (Fig. 1)
sfc
Brake Power
Figure 1. Consumption loop
Of particular significance are point A corresponding to maximum power output and point B corresponding to minimum
specific fuel consumption? It will be realised that point A corresponding to a rather richer mixture than point B, and the
correct (stoichiometric) mixture strength corresponds to a point lying between A and B. The aim in carburettor design is
to endure that over as much as possible of the range of engine power and speed the mixt are supplied to the engine
corresponds with point B, giving best possible economy, while under maximum power full throttle conditions the mixture
corresponds to point A, giving maximum possible power output.
Similar curves may be obtained at part throttle, corresponding to lower maximum power outputs and invariably to higher
minimum specific fuel consumptions. A series of curves taken at part throttle opening are shown in Fig.1. Points C,D and
E correspond respectively to minimum fuel consumption at ¾, ½ and ¼ throttle opening. Points A to E can be replotted
and this will indicate the best performance of the engine that can be obtained the carburettor is adjusted at each throttle
opening to give the best results. A comparison between this curve and that obtained with a carburettor in its standard from
is a measure of the accuracy with which the carburettor characteristics matches the requirements of the engine.
In this test the brake power (PB) will be obtained from the measurements of the engine torque (T) using the electrical
dynamometer
PB 
2 NT
60
[W]
(1)
Where N is the engine rpm.
The specific fuel consumption is
sfc 
consumption of fuel (mass / unit time) m f

brake power
PB
 kg 
 kwh


(2)
Where the fuel consumption mf is determined by measuring the time for the engine to consume a given volume of fuel, say
8 ml
mf 
 f x8 x103
t
 kg 
 sec 
 
(3)
Where the specific gravity of fuel has the value of 0.74 g/cm3 for gasoline.
The air consumption rate will be measured using the viscous flow meter and the calibration curve given before (see the
previous experimet sheets for TD 43 variable compression engine)
PROCEDURE:
1- Calibrate and zero the meter as described in the previous experiment sheet.
2- Adjust the scale on the airflow manometer to read zero.
3- Adjust the compression ratio to 8:1.
4- Fit the 23 mm choke on the carburettor.
5- Start and warm the engine with the mixture screw adjusted at 2.5 turns.
6- Set the engine speed to 1750 rpm at full throttle using the load current control.
7- Decrease the mixture screw setting to a minimum until popping back occurs in the air box. Then increase the mixture
screw setting up to the limit of rich extinction. Note the minimum and maximum mixture screw setting. Choose at least
8 values between these limits.
8- Set the mixture screw to the minimum value.
9- Record readings to torque, fuel and air consumptions.
10- Set the mixture screw to the next higher value. Adjust the load control to bring the engine speed back to 1750 rpm.
11- Repeat steps 10 and 11 until maximum mixture screw setting is reached.
12- Repeat steps 8,9,10 and 11 for 2/3 and 1/3 throttle openings.
RESULTS AND CALCULATIONS:
1- Draw the graph of Air/Fuel ratio against brake power for each throttle openings.
2- Draw the graph of specific fuel consumption against brake power for each throttle openings. (consumption loops)
3- Mark the points corresponding to maximum power output and minimum fuel consumption on each consumption loop
456-
curve.
Find the Air/Fuel ratios corresponding to the points marked in step 3 and tabulate them near the graph drawn in step
2.
Mark the points corresponding to stoichiometric Air/Fuel ratio on the consumption loop curves.
Draw bsfc against Air/Fuel ratio for each throttle opening.
CONCLUSION AND DISCUSSION
1- Comment on the Air/Fuel ratios found for points corresponding to maximum power output and minimum specific fuel
consumption.
2- Theoretically maximum power should be developed at the stoichiometric Air/Fuel ratio ( 14.5 : 1 ). What is the actual
Air / Fuel ratio corresponding to maximum power output in the experiment ? Comment on the discrepancy between
the two values.
3- By looking at the consumtion lop produced state why the popular idea that the carburettor should be adjusted to give
a weak mixture ( corresponding to the part of the curve lying to the left of minimum specific fuel consumption point )
is not true.