Objective
The Objective of the experiment was to determine the fuel consumption of the Chevrolet 4.3 liter engine as a function of coolant temperature, load, and speed using the Latin Squares method and also predict the performance characteristics of a fictitious vehicle based upon engine performance data gathered.
Introduction
Engine testing is an integral part of any engine manufacturing industry. Engines are normally tested by the manufacturing company and sometimes an independent organization to affirm that indeed they perform as per their documented specifications. Most documented d performance ratings however do not specify the conditions under which such performance is achieved. Many manufacturers only give the best performance but without the conditions. In engine testing, however, the conditions must be set and specified to give an accurate indication of exactly how the performance is attained. Engine tests that have been carried out have shown that the rated performances can normally not be achieved under regular operating conditions. Measuring engine parameters is also vital in the creation of more advanced technologies.
According to Gitano (2006)” Testing of engine performance is often important in the development of engine and fuel technologies. Many parameters affect an engine’s performance: the basic engine design, compression ratio, valve timing, ignition timing, fuel, lubricant, and temperature”. Several parameters of engine performance can be measured but the main and most important test is the power test. In this test, the power of the engine is obtained. Power can however not be measured directly; this then implies that other parameters that are directly related to power are measured then used in the computation of power. Power can be computed by obtaining the product of torque and angular velocity.i.e.
Angular velocity in revolutions per minute on the other hand is given by
The two equations can be combined to get
From this formula, we can therefore obtain the power even though it is not measured directly. The torque and angular velocity are measured d and fitted in the above equations to get the power.
Engines can normally be tested in the lab; this is made possible by connecting the engine to some kind of load that strains the engine. In most cases, this loading is provided by a dynamometer which allows the researcher to widely vary the load on the engine. The speed and torque can also, therefore, be varied and measured once the engine is coupled to a dynamometer.
The angular velocity is measured using a device called a tachometer. Most modern tachometers measure the rotational speed optically and display it on a display unit. The more recent digital tachometers have very high accuracies and are easy to use (Testing the Performance of Model Engines, 2005).
Torque on the other hand is measured by applying some force on the engine through the dynamometer. The force is in most cases taken up by a force transducer which gives the measured strain. This strain is amplified to give out a voltage that varies directly with the load. The measured torque is the same as that on the dynamometer.
The throttle position should also be measured. Throttle position sensors are fitted to most engines and this is used to obtain the throttle position and pass its position to the electronic control unit. Throttle position sensors in engines with carburetors can be fixed onto the throttle linkage so as to measure the throttle position directly. In a typical test, the dynamometer is set to a specific speed, and the torque is measured about the throttle position. It is also possible to keep the throttle in one position and vary the speed. Values of varying speed and torque can then be measured and used to compute the power using the above-mentioned relationship (Testing the Performance of Model Engines, 2005).
Fuel consumption can also be measured during the testing as this is a very important parameter. Other parameters that can be measured during the length of the test are airflow and exhaust emissions. Analysis of this can give an idea of just how efficiently the engine is running. Temperatures at several points around the engine may also be measured to establish factors like the optimum operating temperature and if there is overheating. Air/Fuel ratio is another very important factor in engine testing. Different test conditions result in different air-fuel ratios which in turn results in variation in all the above measurable and computable parameters like power, fuel consumption, and overall engine efficiency.
According to Gitano (2006)”Engine testing requires fairly low data-rate measurements because all variables are averaged over several engine cycles. Data acquired in this manner are referred to as “cycle-average data.” Relatively slowly changing values, such as engine speed, fuel flow, engine temperature, and manifold pressure, are measured directly. Torque fluctuates over the cycle of an engine, being highest during the power stroke and lowest during the compression stroke. Fluctuating signals may be filtered electronically to remove this higher-frequency variation, and various digital techniques (such as exponential averaging) may also be applied once the data has been acquired”.
It is worth noting that all instruments used in the experiment must be properly and severally calibrated to ensure that the results are as accurate as possible. Several readings must also be made so as to get values that are consistent and accurate. The several calculations and graphs involved make it necessary to have a proper computer to work out all the calculations accurately (Korcek & Jensen, 2001).
In general, engine testing is vital in mechanical engineering because it helps in the progression of technology continuity in improvement to producing more and more efficient engines. Technologies like variable valve injectors were developed as a means of increasing fuel efficiency in petrol engines after research on fuel efficiency using such tests. Engine models are also revised after findings in engine testing leading to the development of more efficient systems (Büchi & Freunberger, 2007). High-performance engines used in sports cars have also been developed after much research and engine testing. Combined power and fuel efficiency testing is key in helping develop engines that strike a good balance between high power and optimum efficiency. Exhaust emissions and their effect on the environment have been a very contentious issue with the ever-increasing environmental awareness. Engine testing can obviously not be ignored in this regard because fuel-run engines are major pollutants of the environment and everything must be done to ensure that the effect of these emissions is reduced or eliminated altogether (Laser & Larson, 2009).
Procedure
Equipment
The equipment used in these experiment included:
- Chevrolet 4.3 liter V -6 engine, Dyno-mite dynamometer with computer data acquisition system
Methodology
Fuel consumption for torques of 40, 80, 120, 160 ft-lbs, Engine speeds of 1800, 2400, 3000, 3600 rpm, coolant temperatures of 175, 185, 195, 205, and O F were measured in accordance with factorial testing procedures. The fuel consumption rate for one extra point not in the Latin squares arrangement was also measured. The sweep function allowed the dyno operator to select a pre-programmed load to allow the engine to operate through the allowable engine speeds. The maximum safe speed allowed in this test was 4000 rpm. We set the lower limit of the engine speed to 1600 rpm and the upper limit to 4000 rpm. The rate of engine speed increase was 200 rpm/sec. After setting the lower rpm, we increased the engine speed slowly to allow the dyno to load and maintain speed. Once the throttle setting was finalized, we engaged the sweep test. The data from previous experiments had resulted in the following relation for torque in foot-lbs and engine speed in rpm:
The Figure below gives a schematic of the apparatus:
- Torque = 3Xl0-9rpm3 -5Xl0-5 rpm2 +0.22rpm -40.974
The data gathered allowed us to construct the power and torque curves for the engine to be used to predict the performance of the vehicle. The testing was performed at a coolant temperature of 205 F.
Fuel Calibration
Tests conducted in previous years showed that the fuel calibration of this dynamometer was off. The screen displayed the amount of fuel being consumed by the engine as the difference between the amount of fuel pumped from the fuel tank (meter a) and the amount of fuel being returned to the tank (meter b). The formula used to correct the fuel flow rate is as follows:
- Fuel Consumed = -0.0031×3 + 0.2055×2 – 2.5919x + 18.767
Where x was taken the fuel consumption difference of (a-b) from the data files. After recording the fuel usage from the computer files, we applied the correction factor to account for the deviation. Please note that the units were in lbmlhour.
Methodology for vehicle performance calculations
The assignment for the performance testing of the Chevrolet 4.3 liter V-6 engine was to predict the vehicle performance based upon the data collected from the engine and apply established engineering principles to determine the characteristics of a fictitious vehicle.
To predict the acceleration rates of the vehicle, we determined the force required to accelerate the vehicle based upon F=ma. At any given instant, the forces must be in equilibrium. Thus, if a vehicle is at cruise conditions, the force transmitted by the drive wheels to the roadway should be equal to the drag force on the car exerted by the vehicle’s motion through the air and the rolling resistance which is a function of tire construction and the weight of the vehicle. Under acceleration, the force available to accelerate the vehicle is the difference between the required force to maintain the vehicle at the present velocity and the force necessary to accelerate the vehicle, F-ma.
In this experiment, we collected the performance data for full-throttle operation. By utilizing the transmission and final gear ratios along with the rolling radius of the tires, we were able to determine the propulsive force at any given instant. For purposes of this experiment we assumed that the parasitic power loss for the engine and drive train is given by the following expression:
- PowerparaSiliC = 3XIO-5(V3) – 0.0065(V2) + 0.5505(V) + 17.089
Where: power was in horsepower and V is in mph
The rolling resistance power requirement is given by the expression:
- Power rolling = 0.0083(V2) + 0.1419(V) + lXl 0-11
Where: power was in horsepower and V is in mph
For fuel economy calculations at a constant speed, we determined the power requirements and compared them with the fuel usage as determined by actual testing.
Reference list
Büchi , F. N., & Freunberger S. A. (2007).On the Efficiency of an Advanced Automotive Fuel Cell System. Fuel Cells 7(2), 159-164.
Gitano, H.(2006).Small Engine Dynamometer Testing. Performing Dynamometer Testing on Combustion Engines. Web.
Korcek, S., & Jensen, K. (2001).Maximizing the fuel efficiency of engine oils: The role of tribology. Tribotest 7(3), 187-201.
Laser, M.,& Larson, E.(2009). Bruce Dale Comparative analysis of efficiency, environmental impact, and process economics for mature biomass refining scenarios. Biofuels, Bioproducts and Biorefining 3(2), 247-270.
Suresh, A. V., & Mehta, A. K. (1993).A new test technique for the laboratory evaluation of energy-efficient engine oils. Lubrication Science, 5(4), 283-294.
Testing the Performance of Model Engines. (2008). Web.