Energy Conversion and Management 48 (2007) 961–966 www.elsevier.com/locate/enconman
Intake plenum volume and its influence on the engine performance, cyclic variability and emissions M.A. Ceviz
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Department of Mechanical Engineering, Faculty of Engineering, University of Atatu¨rk, Erzurum 25240, Turkey Received 22 July 2005; accepted 9 August 2006 Available online 10 October 2006
Abstract Intake manifold connects the intake system to the intake valve of the engine and through which air or air–fuel mixture is drawn into the cylinder. Details of the flow in intake manifolds are extremely complex. Recently, most of engine companies are focused on variable intake manifold technology due to their improvement on engine performance. This paper investigates the effects of intake plenum volume variation on engine performance and emissions to constitute a base study for variable intake plenum. Brake and indicated engine performance characteristics, coefficient of variation in indicated mean effective pressure (COVimep) as an indicator for cyclic variability, pulsating flow pressure in the intake manifold runner, and CO, CO2 and HC emissions were taken into consideration to evaluate the effects of different plenum volumes. The results of this study showed that the variation in the plenum volume causes an improvement on the engine performance and the pollutant emissions. The brake torque and related performance characteristics improved pronouncedly about between 1700 and 2600 rpm by increasing plenum volume. Additionally, although the increase in the plenum volume caused the mixture leaner due to the increase in the intake runner pressure and lean mixtures inclined to increase the cyclic variability, a decrease was interestingly observed in the COVimep. 2006 Elsevier Ltd. All rights reserved. Keywords: Intake manifold; Intake plenum; Engine performance; Cyclic variability; Emissions
1. Introduction Intake manifolds consist typically of a plenum, to the inlet of which bolts the throttle body, with the individual runners feeding branches which lead to each cylinder. Important design criteria are: low air flow resistance; good distribution of air and fuel between cylinders; runner and branch lengths that take advantage of ram and tuning effects; sufficient (but not excessive) heating to ensure adequate fuel vaporization with carbureted or throttle-body injected engines [1]. The intake system on an engine has one main goal, to get as much air–fuel mixture into the cylinder as possible.
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0196-8904/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.08.006
The design of the intake system commonly implies a complex process of synthesis which is mainly based upon experimentation on the engine test bench. A great part of the bibliography concerning this subject was given in [2–5]. The pressure in intake manifold varies during each cylinder intake process due to the piston velocity variations, valve open area variations, and the unsteady gas flow effects that result from these geometric variations. The mass of air inducted into the cylinder, and hence the volumetric efficiency, is almost entirely determined by the pressure level in the intake port during the short period before intake valve is closed [1]. During induction, and as the piston reaches its maximum speed, the pressure immediately upstream of the valve reaches its minimum value. This initiates a refraction wave which travels upstream in the inlet duct to be reflected as a compression wave at its open end. Tuning occurs when this
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compression wave arrives back at the valve when it is closing (IVC) [3]. The behaviour of the inlet duct can be modelled as that of a Helmholtz resonator or even, in simple cases, as a quarter-wave pipe. Margary et al. [3] investigated the effect of intake duct length on volumetric efficiency and in cylinder flow field in a four-stroke single cylinder research DI Diesel engine, motored at 1000–3000 rpm. They considered three lengths of straight duct upstream of the helical inlet port of the engine, and reported measurements of instantaneous mass flow rate and pressure drop across the port as a function of duct length and engine speed. The results show a significant increase of swirl velocity magnitudes as resonance of the induction system is reached. The pressure variations generated by the pulsating flow can be used to improve the intake pressure level by configuring the intake manifold optimizing the pressure pulses in the intake system. Thus, a static intake manifold can only be optimized for one specific rpm, so it is beneficial to develop a method to vary the intake length and/ or volume. In recent years, variable intake manifold became increasingly more popular, which improves torque delivery at low speed without hurting high speed power. Most designs employ two intake manifolds with different length because of the difficulty of producing continuously variable intake manifold. Of these manifolds, the longer one is used for low-rpm, and the shorter one for high-rpm. Some of the engine manufacturers (i.e. Mazda, BMW) successfully used variable intake manifold. Kro¨mer et al. [6] studied on Audi V6 engine to improve torque at low engine speed, and generated a broader torque hump in the commonly used speed range between 2000 and 3500 rpm. They also produced a high power output by using two intake manifolds with different lengths and cross-sectional areas. Recently, Christie and Frank [7] investigated a simplified variable intake manifold for use with an internal combustion engine. The manifold utilizes a single valve actuator to vary the resonant frequency of a pair of plenums. The two plenums are joined by first and second communication passages of differing lengths connected to provide cross flow between two plenums. Each plenum has an inlet for receiving intake air from a throttle body and a group of runners adapted for connection with a like group of engine cylinders to direct inlet air from the plenums to combustion chambers of the cylinders. Narayanaswamy [8] studied on an intake manifold includes a permanent runner which provides airflow communication between a plenum volume and a plurality of engine cylinders. An active runner assembly retracts into and extends from the permanent runner assembly in response to a drive system which communicates with a controller to selectively change the length of the intake manifold assembly to optimize performance. Whilst there are several experimental and theoretical researches recently in the field of design of optimal intake
system and variable intake manifold, there are a few published experimental data related with inlet plenum. Shaw et al. [9] investigated the effect of plenum-runner interface geometry. They measured experimentally the steady flow through a plenum-runner system within an inlet manifold and also predicted with computational fluid dynamics (CFD). They found in particular that the losses due to different interfaces at the plenum-runner junction have considerably affected on the steady flow cases. Jawad et al. [10] developed a new induction system for use in the 2001 Formula SAE competition. A 600 Honda CBR F4 four-stroke engine intake was designed using dual plenums, which prevents charging losses due to overlapping intake events at low engine speeds. Dual butterfly valves actuated at high engine speeds enable plenum volume combination for improved high-end performance. Their design increased significantly the peak horsepower. In this paper, the effects of intake plenum volume variation on engine performance and emissions were studied experimentally to constitute a base for continuously variable intake plenum design since obtaining a continuously variable intake plenum is simple using the engine speed data from a running engine. 2. Experimental apparatus and test procedure The engine test bed was explained in the previous studies of the author [11,12], which consists of a control panel, a hydraulic dynamometer and measurement instruments. The experimental setup is shown in Fig. 1 and the engine specification is summarized in Table 1. For measuring the fuel flow rate, a P8126 type, Cussons compuflow gravimetric fuel flow meter was used. A VLT 3040 gas analyzer having electrochemical sensors was used to measure the CO, CO2, and HC emissions. In-cylinder pressure was measured by a piezo-electric pressure transducer (KISTLER, 6117BFD17 type) and collected by the acquisition card at the rate of 100 kHz on a personal computer. Additionally, for the pressure measuring at the intake runner of cylinder used for measuring in-cylinder pressure, a manifold absolute pressure sensor (Delco Electronics 1 BAR MAP sensor) was used. This sensor has proven extremely reliable in off-road and marine environments as well as highway usage. General specifications of the sensor are presented in Table 2. The sensor has 6.0 ms response time and produced for naturally aspirated engines. The measuring point was at the distance of 6.5 cm upstream intake valve. The individual contributions to the uncertainties of the parameters for each of the measured physical properties and the maximum uncertainties of the derived experimental parameters are summarized in Table 3. The maximum uncertainties are determined using the estimation method of Kline and McClintock [13]. When calculating the indicated engine characteristics, the averaged value of in-cylinder pressure signal and corresponding intake runner pressure signal for 50 consecutive
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8 9
20 7
4 6 5
17
3
19
1 15
2
16 18
12 13 14 10
11 Fig. 1. Schematic diagram of experimental apparatus. 1 – Engine; 2 – hydraulic dynamometer; 3 – fuel flow meter unit; 4 – gravimetric fuel flow meter; 5 – additional plenums; 6 – carburetor; 7 – air surge tank; 8 – air flow meter probe (hot wire); 9 – air flow meter; 10 – exhaust gas analyzer; 11 – muffler; 12 – shaft encoder; 13 – spark plug mounted piezoelectric transducer; 14 – ignition pick-up; 15 – spark plugs; 16 – distributor; 17 – MAP sensor; 18 – charge amplifier and oscilloscope; 19 – analog to digital converter; 20 – personal computer.
Table 1 Specification of the test engine Bore (mm) · stroke (mm) Compression ratio Inlet valve opens (btdc) Inlet valve closes (abdc) Exhaust valve opens (bbdc) Exhaust valve closes (atdc) Maximum power Maximum torque Cooling system
86.4 · 67.4 9.2 7 35 37 5 62 kW at 5800 rpm 13 da Nm at 2900 rpm Water-cooled
Table 2 Specification of the MAP sensor Supply voltage Over voltage Reverse polarity Supply current Response time Minimum load
4.9–5.1 VDC 16 VDC MAX 16 VDC MAX 5 mA MAX 6.0 ms MAX 9.0 K X(pull up or pull down)
cycles were used to eliminate the effects of the cyclic variations. Calculation of indicated mean effective pressure and COVimep were described in the author’s previous study [11]. To reduce the amount of high frequency noise on pressure signal, low pass digital filtering was applied to all the pressure data. Cylinder pressure data is pegged by assuming the pressure at bottom dead centre after the intake stroke is equal to the mean intake manifold pressure [14]. For all
Table 3 Accuracies of the measurements and the uncertainties in the derived experimental parameters Measurements
Accuracy
Load Speed Fuel consumption Intake manifold absolute pressure CO CO2 HC Relative air ratio Heating value
±0.1 kg ±1 rpm ±0.1 g ±1 kPa ±0.01% vol. ±0.01% vol. ±1 ppm ±0.001 ±1%
Derived experimental parameters
Maximum uncertainty (%)
Torque Power Mean effective pressure Specific fuel consumption Thermal efficiency
±5.0 ±5.0 ±5.0 ±5.2 ±5.3
the experiments, ignition timing was adjusted to maximum brake torque timing (MBT). After the engine reached the steady-state conditions, the first experiment was conducted with original intake manifold. The second and third experiments were carried out with separately 90 cm3 and 180 cm3 plenum additions, respectively. There was a reverse effect on engine performance after attaching additional volume from this point. The engine speed were regulated by changing the throttle valve opening position, and test matrix consisted of 5 speeds near 1250, 1500, 2000, 2500, and 3000 rpm steps
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for each operation on intake plenum addition. The additional plenums had the geometries suitable for entrance of the original intake manifold, and located among the carburetor and manifold. 3. Results and discussion Figs. 2–6 show the brake and indicated engine characteristics. Fig. 2 presents an influence of intake plenum on the brake torque. Variation in intake plenum volume gen-
600 550 Specific Fuel Consumption [g/kW.h]
964
500
No addition
450
180 cm3 plenum addition
90 cm3 plenum addition
400 350 300 250 200 150 1000
11 10
Brake Torque [kg.m]
9
1500
2000
2500
3000
3500
Engine Speed (rpm)
180 cm3 plenum addition 90 cm3 plenum addition No addition
Fig. 5. Variation of specific fuel consumption with engine speed for three different intake plenum volumes.
8 7 6 5
0.45
4 3
0.40
1 1000
1500
2000
2500
3000
3500
Engine Speed (rpm)
Fig. 2. Variation of brake torque with engine speed for three different intake plenum volumes.
Thermal Efficiency [-]
2 0.35 0.30 180 cm3 plenum addition 90 cm3 plenum addition
0.25
No addition
0.20 0.15
Brake and Indicated Power [kW]
40 35
180 cm3 plenum addition 90 cm3 plenum addition No addition
30
0.10 1000
Indicated
2000
2500
3000
3500
Engine Speed (rpm)
25
Fig. 6. Variation of thermal efficiency with engine speed for three different intake plenum volumes.
20 Brake
15 10 5 0 1000
1500
2000
2500
3000
3500
Engine Speed (rpm)
Fig. 3. Variation of brake and indicated power with engine speed for three different intake plenum volumes.
Brake and Indicated Mean Effective Pressure [kPa]
1500
1000 900 800 700
180 cm3 plenum addition 90 cm3 plenum addition No addition
Indicated
600 500 Brake
400 300 200 100 0 1000
1500
2000
2500
3000
3500
Engine Speed (rpm)
Fig. 4. Variation of brake and indicated mean effective pressure with engine speed for three different intake plenum volumes.
erated a high torque when running with additional plenums, but there is a broader and high torque hump with 180 cm3 plenum addition in the commonly used speed range about between 1700 and 2600 rpm for this type of engine. After the 2600 rpm, the effect of the increase in volume on the intake pressure level became worse and decreased the engine performance characteristics. Figs. 3 and 4 show similar effect on engine brake and indicated power output and mean effective pressure. Increase in brake torque caused an increase in the power output at near the same engine speed. Figs. 5 and 6 show specific fuel consumption and engine thermal efficiency variations, respectively. It can be seen from these figures, there is an improvement in engine efficiency with the use of the additional plenums, however, a pronounced reverse effect was observed on these parameters when using 180 cm3 plenum addition after near 2500 rpm. This effect can be seen from all other figures. Main reason of above mentioned variations in engine performance characteristics was the increase in the intake manifold pressure. Due to this increase, more air entered into the cylinder, in this way the relative air ratio increased. The increase in the intake manifold pressure can be seen
M.A. Ceviz / Energy Conversion and Management 48 (2007) 961–966 0.040
70
0.035
60
0.030 COVimep [-]
Intake Manifold Absolute Pressure [kPa]
80
965
50 180 cm3 plenum addition
40
90 cm3 plenum addition No addition
0.025
0.020
30 20 1000
No addition 90 cm3 plenum addition 180 cm3 plenum addition
0.015 1500
2000
2500
3000
3500
Engine Speed (rpm)
0.010 1000
Fig. 7. Variation of intake manifold absolute pressure with engine speed for three different intake plenum volumes.
1500
2000
2500
3000
3500
Engine Speed (rpm)
Fig. 10. Variation of coefficient of variation of indicated mean effective pressure with engine speed for three different intake plenum volumes. Intake Opens
Intake Closes
TDC
95
180 cm3 plenum addition
Intake Period
6.2
75
65
55
180 cm3 plenum addition
4.2 3.2 2.2
45
1.2
No plenum addition
35 -400
-300
-200
-100
0
100
200
300
400
Crank Angle (Degree)
1.25 1.20 180 cm3 plenum addition 90 cm3 plenum addition No addition
1.15 1.10 1.05 1.00 0.95 0.90 1000
1500
2000
0.2 1000
1500
2000
2500
3000
3500
Engine Speed (rpm)
Fig. 8. Variation of intake manifold pressure for three different intake plenum volumes.
Relative Air Ratio [-]
No addition 90 cm3 plenum addition
5.2 CO [% Vol.]
Inlet Manifold Pressure [kPa]
7.2
90 cm3 plenum addition
85
2500
3000
3500
Engine Speed (rpm)
Fig. 9. Variation of relative air ratio with engine speed for three different intake plenum volumes.
from Figs. 7 and 8. Fig. 8 shows the pressure distribution at the intake runner at near 2000 rpm engine speed. Additionally, Fig. 9 shows the variations in the relative air ratio. Coefficient of variation in indicated mean effective pressure, COVimep, is one of the important measures of cyclic variability, derived from pressure data, and the cyclic variation is specially a problem for lean burn operating engines. Cyclic variability is recognized as a limit for oper-
Fig. 11. Variation of CO emission with engine speed for three different intake plenum volumes.
ating conditions with lean and highly diluted mixtures. Previous studies showed that if cyclic variability could have been eliminated, there would be a 10% increase in the power output for the same fuel consumption and power pollution of emissions from the engine [15]. The variation in the coefficient of variation in indicated mean effective pressure can be seen from Fig. 10. The decrease in the COVimep by using the additional plenum is an interesting result because of the increase in the relative air ratio at the corresponding engine speeds. As the mixture leaned, there is an expectation in the increase in the cyclic variability due to the deterioration of the mixture homogeneities, and decrease in the engine power output; however it can be inferred from Figs. 3, 9 and 10 that, especially at 1700–2600 rpm, the quality of air–fuel mixture homogeneity increased. Figs. 11–13 show the effect of the plenum addition on the CO, CO2 and HC emissions, respectively. It can be seen from these figures that the plenum addition caused a decrease in the HC and CO emissions, and an increase in the CO2 emissions. The decrease is more significant up to 2600 rpm. Main reason of this improvement was the better combustion when using additional plenums due to the increase in the relative air ratio and the mixture quality.
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CO2 [% Vol.]
17 16 15 14 180 cm3 plenum addition 90 cm3 plenum addition No addition
13 12 1000
1500
2000
2500
3000
3500
2. When the intake plenum volume increased, intake manifold pressure increased, and mixture became leaner. Additionally, COVimep was decreased despite negative effect of the leaner mixture. 3. Pollutant emissions decreased due to the increase in the relative air ratio and the mixture homogeneities. 4. It can be inferred that the intake plenum volume is highly effective on engine performance characteristics and emissions. The engine performance can be increased by using intake plenum volume that is continuously variable.
Engine Speed (rpm)
Fig. 12. Variation of CO2 emission with engine speed for three different intake plenum volumes.
600 550
HC [ppm]
500
No addition 90 cm3 plenum addition 180 cm3 plenum addition
450 400 350 300 250 200 1000
1500
2000
2500
3000
3500
Engine Speed (rpm)
Fig. 13. Variation of HC emission with engine speed for three different intake plenum volumes.
This effect continued up to about 2600 rpm and after this point became worse due to the reverse effect of volume addition on the pulsating air–fuel flow. 4. Conclusions From the study given in this paper, the followings can be deduced: 1. Increase in the plenum volume improved the engine performance greatly between 1700 and 2600 rpm. However, as the engine speed exceeded from 2600 rpm, volume must be decreased to optimize the engine performance for the test engine.
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