A Study on a mechanical continuously variable injection timing system for improvement of agricultural small diesel DI engine

This paper presents the development of a MCVIT (mechanical continuous variable injection timing) system for evaluating effects of injection timing – one of the efficiently experimental methods for improving diesel engine performance and emission characteristics to match modern emission regulations and bio-fuels. A MCVIT system has been designed and built with the ability to adjust freely and directly the injection timing for a wide range from 0 to 40°CA BTDC (before top dead center) while keeping the same injection pressure rate. Some experiments have been done to verify its significant specifications such as friction torque – speed curve, accuracy and stability. The results show that the maximum friction torque of this system is around 2.6N.m over range of engine speed and its maximum deviation is ±1.0°CA over a large range of testing injection timing. Preliminary study on VIKYNO RV215-2 agricultural small diesel DI engine has also proved that the engine performance and emission characteristics are directly influenced by injection timing. Thus the developed MCVIT system is an efficient and low cost tool for R&D activities in small diesel engines.


INTRODUCTION
VIKYNO RV125-2 engine -water cooled 12.5HP single cylinder diesel DI engine -is the most popular agricultural small diesel DI engines for Vietnamese rural applications such as electricity generation, stationary power-trains and agricultural vehicles.Under very old designed license of Kubota ® , the engine performance characteristics were unsatisfied.Thus, its performances should be enhanced to fulfill the emission regulations for non-road engines such as US TIER 2 or STAGE II adopted by many target markets.Preliminary, the basic performance characteristics of that engine were reported and analyzed [1].It was found that its exhaust emission is unsatisfied for TIER2 standard.Many measures [2][3][4][5][6] were studied to improve the performance characteristics of small diesel DI engines such as combustion chamber geometry modification, intake and exhaust system re-design, increase of injection pressure, and utilization of split injection systems.In this paper, the effects of injection timing's change on the engine performances are proposed to study.
Unlike distributor-injection pump and electrically controlled injectors, the injection timing of agricultural small diesel DI engine is unable to change freely.This fact demands to develop an injection control system that allows changing continuously the injection timing in a wide range.Also the system must has the same injection pressure rate, fast response, continuously tuning capability, low friction torque, high accuracy and stable operation.Such kind of system is not commercially available.In this study, a MCVIT system combined with an electronic injection timing checking system is successfully developed.The measure of injection timing's change has the benefits of time-saving, low-cost and suitable for agriculturally purposed engines.It is expected that the optimized injection timing could be better for both engine performance and emission characteristics.
Thus, this paper aims to demonstrate the MCVIT system and its characteristics and to evaluate the basic performance characteristics of the test engine with and without MCVIT system.Preliminary the effects of injection timing on performance and emission characteristics RV125-2 engine are also analyzed.

DESIGN OF MCVIT SYSTEM
A full MCVIT system consists of a cam phaser to adjust injection timing, an extra fuel delivery system and an electronic injection timing checking system.

Principle of MCVIT system
The MCVIT design concept is based on continuously variable cam phasing method.The original fuel delivery system is shut off and fuel is supplied to injector by an external fuel delivery system which uses a set of the same kind of camshaft, diesel-injection pump and high pressure line.Actually, original diesel-injection pump and high pressure line will be re-used as extra parts to keep exactly the same injection pressure rate and fuel delivery quantity.
As illustrated in Fig. 1, the extra dieselinjection pump is driven by the extra camshaft which is driven directly by timing gear of a pseudo crankshaft driven from engine flywheel via a cam phaser Fig. 3.The cam phaser consists of a timing belt, a pair of vertically movable pulleys and 04 fixed pulleys as belt guiders.The movable pulleys are a pair of 28-tooth gears that move in opposite directions.The displacement of movable pulleys (Δx) produces relative angular between engine crankshaft and the pseudo crankshaft (Δθ) resulting in an injection timing change of the same value, i.e.Δθ ( ).The upward and downward displacements of movable pulleys results in advanced injection timing and retarded injection timing respectively.
There are some advantages of this design concept: (1) Injection timing, depending on presettings of the system and the displacement of movable pulleys can be varied in a wide range and fine tuning; (2) Injection timing can be changed directly with fast response during engine operation; (3) Easy to build and no requirement of highly precise machining methods.Th design brake t (2) the and it is at a con problem tensione injectio can cau the incr well.There are 03 experiments which have been done to evaluate actual specifications and functions of the developed MCVIT system.

Test 1: Friction loss determination and fuel delivery check
The engine runs at 50% of load with constant speeds being set up on the range from 1,200 to 2,400rpm by a step of 200rpm.Three test cases will be done, including: (1) disconnect the MCVIT system from the engine and use engine original fuel delivery system; (2) connects the MCVIT system to the engine but use engine original fuel delivery system; and (3) shut off engine original fuel delivery system and use the MCVIT system with full functions.The same injection timings are set up in the case 1 & 2.
The results of case 1 and 2 are used to determine the friction torque of MCVIT system which is a part of engine brake torque.It should be noted that engine brake torque (hereafter shortly called brake torque) is equal to dynamometer brake torque (hereafter shortly called measured brake torque) plus the friction torque of MCVIT system.The results of case 2 and 3 are used to evaluate the machining and assembly errors of MCVIT system effects on the injection pressure rate and fuel delivery quantity.

Test 2: Accuracy and stability check
Engine runs at 50% of load and 1,800rpm with injection timings being set up from TDC to 30°CA BTDC.The real-time injection timing is recorded to evaluate the accuracy and stability of MCVIT system.

Test 3: Preliminary study on the effect of injection timing on engine performance and emission characteristics at constant load and engine speed
The engine runs at 50% of load and 1,800 rpm with injection timings being set up at 4, 8, 10, 12, 14, 16, 20 and 24°CA BTDC.Brake torque, brake power, brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), NOx and opacity are measured and analyzed in accordance with change of injection timing.

Test result 1: Friction loss determination and fuel delivery check
As shown in Fig. 6, operating at the same fuel delivery quantities and injection timing, engine performances for case 2 and 3 are found to be similar over the whole engine speed range.Maximum deviations of measured brake torque and power are around 0.4N.m and 86 W respectively.These results show that MCVIT system could be able to keep the original injection pressure rate and fuel delivery quantity.This is a very important factor to ensure the same engine performance and emission characteristics with and without MCVIT system.
The deviation of measured brake torque between case 1 and 2 is used to determine the curve of friction torque and speed of a MCVIT system with an operating range (Fig. 7).Due to short speed range, this curve can be approximated by a linear equation as shown in Eq. 1. Eq. 1 will be used to correct measured brake torque and power to actual engine brake torque and power which are required for calculating brake specific fuel consumption and brake thermal efficiency.
As shown in the figure, additionally, the friction loss that was caused by the MCVIT system, is estimated over engine torque in percentage (%).It is found that the relative friction loss is less than 7% for whole range of engine speed.This confirmed that the MCVIT system has a small effect on the brake torque.

Test result 2: Accuracy and stability check
Fig. 8 illustrates accuracy and stability of injection timing over a wide range up to 40°CA BTDC.For each specific injection timing, maximum deviation is found to be around ±1.0°CA.This may be caused by the vibration in single cylinder diesel engine and the elasticity of timing belt.Especially, highly advanced injection timing causes too much engine vibration and can lead to high noise injection timing detection.Fast response and fine tuning ability are also illustrated in Fig 8.

Test result 3: Preliminary study on effect of injection timings
The effects of injection timing on engine performance characteristics are analyzed and shown in Fig. 9 & Fig. 10.When injection timing is advanced from 4 to 24°CA BTDC, brake torque, brake power and BTE show the same trends and all reach their peak value at around 120CA BTDC.Respectively, BSFC obtains the lowest value at the same injection timing.In addition, NOx emission increases rapidly from about 500ppm to above 2,800ppm.In contrast, opacity decreases drastically from above 7% to below 1%.The above-mentioned results show that at 1800 rpm injection timing from 8 to 14°CA are good for engine performance due to perfect match of, mixture preparation, combustion rate and thermodynamic processes in the combustion chamber.In case that more advance in injection timings (above 14°CA BTDC) will ignite the combustion sooner.This causes rapidly increasing of temperature and pressure in the combustion chamber, especially when the piston is moving toward combustion TDC.As the result of this, combustion energy loss increases; and brake torque, brake power, brake thermal efficiency and BFSC are all reduced.Higher combustion temperature and pressure are also good conditions to produce NOx.In contrast, opacity is reduced because fuel is burned easily and there is enough time for complete combustion.More retarded injection timings (below 8°CA BTDC) could lead to later starting point of combustion.Combustion process takes place mostly when the piston is running away from TDC.In this case, the volume of the combustion chamber increases fast and causes the reduction in combustion temperature.As a result, it reduces thermal efficiency and thus reduces brake torque, brake power and BSFC as well.The lower combustion temperature generates, the lower NOx obtains.However, lower combustion temperature and higher time loss of incomplete combustion has increased opacity emission.
It is clear that the change of injection timing has the direct effects on engine performance and emission characteristics of a diesel DI engine.

CONCLUSIONS AND FURTHER STUDIES
A newly mechanical continuously variable injection timing system is properly designed for small diesel DI engines that could be operated with high stability and reliability.The preliminary results show that: (1) While operating with MCVIT system, the deviations of measured brake torque and power are only max.

Fig. 6
Fig.6 Friction loss determination and fuel delivery check