Original Article

The influence of the crankshaft offset on the piston position, the indicated specific fuel economy and the emissions of a direct-injection diesel engine

Proc IMechE Part D: J Automobile Engineering 2014, Vol. 228(5) 500–509 Ó IMechE 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954407013501159 pid.sagepub.com

David MacMillan1, Theo Law1, Paul J Shayler1 and Ian Pegg2

Abstract The effect of offsetting the crankshaft from the cylinder centre-line by 20 mm towards the thrust side was investigated. The effect on the piston top-dead-centre and bottom-dead-centre positions, the valve timing requirements, the indicated performance and the emissions was examined. The investigation was carried out on a common-rail direct-injection diesel engine. The compression ratio was maintained constant, but the piston’s top-dead-centre position and the piston’s bottom-dead-centre position occur 5.5° crank angle later and 10.5° crank angle later respectively than for the zerooffset case. Camshaft position changes were used to retain either the same valve overlap or the same inlet and exhaust valve opening times as for the zero-offset case. The results show that the indicated performance changes are small and that the direction of change can vary with the operating condition. The largest changes in the nitrogen oxide emissions were consistently increases; the largest changes in the carbon monoxide emissions and the filter smoke number were consistently reductions. Small increases in the indicated specific fuel consumption occurred in the low-load cases and a larger reduction occurred in one of the high-load cases. The indicated mean effective pressure fell in the low-load condition and increased or was unchanged in the high-load condition. Overall, the effect of the offset on the indicated performance is minor and secondary to the effect on the rubbing friction; the results are similar when either the valve overlap or the valve opening time was maintained when the crankshaft was offset.

Keywords Diesel engine, offset crankshaft, indicated performance, emissions, fuel economy

Date received: 29 October 2012; accepted: 17 July 2013

Introduction A number of original equipment manufacturers have produced engine designs with an offset crankshaft. Recent examples include the 1.0 l, three-cylinder gasoline engines described by Friedfeldt et al.1 and Lee et al.2 For light-duty automotive applications, the offset is typically 8–15 mm towards the thrust side, reducing the side thrust acting on the piston and altering the dependence of the piston position on the crank angle (CA). The main benefit is commonly presented to be a reduction in the rubbing friction at the piston liner interface due to the former.3–9 Less attention has been given to quantifying the effect on the cylinder indicated conditions of the latter, although some investigators have reported positive benefits in the form of an improved thermodynamic efficiency4 and reductions in the indicated specific emissions. These effects on the indicated performance were the subject of the investigation

reported in the following. The investigation was carried out on a single-cylinder direct-injection diesel engine with a high-pressure common-rail injection system. The engine crankshaft could be offset by up to 20 mm; the results for this case were compared with the results for the baseline case when the crankshaft has zero offset. Most previous investigations have focused on the potential to reduce the friction by offsetting the crankshaft.3–9 Friction losses in the piston assembly can 1

Department of Mechanical, Materials and Manufacturing Engineering, Faculty of Engineering, University of Nottingham, Nottingham, UK 2 Ford Motor Company, Dunton Technical Centre15/2B-A06-A, Basildon, UK Corresponding author: Paul J Shayler, Department of Mechanical, Materials and Manufacturing Engineering, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK. Email: [email protected]

MacMillan et al. contribute as much as two thirds of the total rubbing friction losses in an engine.3, 9 Offsetting the crankshaft to the thrust side relative to the cylinder bore reduces the piston’s side force for a given cylinder pressure during the expansion stroke but increases it during the compression stroke. As pressures during compression are lower than during the expansion stroke, the net effect has been shown to be a reduction in the overall frictional force opposing movement of the piston and hence friction work.3–5 The benefit depends on the ratio of the offset magnitude to the connecting-rod length.4 The optimum offset is reported to vary with the engine geometry,3, 4 the engine speed and the engine load, with larger offset magnitudes beneficial at lower engine speeds.3 Increasing the offset alters the connecting-rod angle relative to the cylinder axis, with the angle decreasing during the first half of a revolution, i.e. during the induction and power strokes, and increasing during the second half, i.e. during the compression and exhaust strokes. A crankshaft offset also modifies the point in the cycle where the piston moves from the thrust side to the anti-thrust side.5, 6 Increasing the offset acts to retard this movement from around the top dead centre (TDC) to the middle of the expansion stroke.5 In addition, the sliding speed, the oil-film thickness and the lubrication regime of the piston are modified.3, 6 The friction benefit during the expansion stroke must outweigh the penalty of increased friction during the compression stroke for an overall gain.5, 6 Although improvements of up to 3% in the realworld fuel economy have been reported for a spark ignition engine,4 it is not clear how much of this, if any, could be attributed to an improved indicated performance, and little has been published regarding the emission responses or the importance of the cam timing. Particular attention was paid to these in this paper, together with the air flow characteristics and the thermodynamic analyses. Jibben10 noted that the increase in the intake stroke produced by offsetting the crankshaft gives more time for induction and improved engine breathing. He concluded that the most significant improvement would be more complete combustion, although no experimental data were examined and the degree to which the crank offset influences combustion remained unclear. In studies carried out on spark ignition engines, Sano et al.4 noted that the degree of constant-volume combustion increased with increasing offset as the piston dwelled longer around the TDC, while Shin et al.7 reported no significant differences in combustion with the offset.

Experimental facilities and test methodology Engine set-up The investigation was carried out on a Ricardo Hydra single-cylinder diesel engine with a swept volume of about 550 cm3 and features which allowed the

501 crankshaft to be offset by up to 20 mm. The engine had a Ford cylinder head with a combustion system and fuel injection equipment specifications to meet the EuroV emissions requirements. The engine was run without external exhaust gas recirculation to maximise the sensitivity of the responses to the thermodynamic changes rather than to the imposed dilution effects. The engine was set up in the naturally aspirated form but with a facility able to supply compressed intake air in lieu of a turbocharger. The compressed air path incorporated a 100 l pressure vessel to dampen pressure fluctuations. When operating in low-speed low-load conditions, the engine ran naturally aspirated. The exhaust back pressure was controlled using a steppermotor-actuated butterfly valve immediately downstream of the exhaust manifold. The intake pressure regulator and backpressure stepper motor were controlled through the National Instruments data acquisition system to achieve target pressures at bottom dead centre (BDC) on the intake stroke. The engine was instrumented with K-type thermocouples to record the oil temperature, the coolant temperature and the intake air temperature; these were controlled with auxiliary heaters. The fuel temperature was maintained at 20 °C, and the fuel consumption was measured using a positive-displacement flow meter. The engine-out emissions were measured with a Horiba MEXA-7100 DEGR exhaust gas analyser system. An AVL 415 smoke meter was used to record the filter smoke number (FSN). The air-to-fuel ratio (AFR) was calculated from the oxygen balance using the measured exhaust gas composition as defined by Heywood.11 The cylinder pressure was measured using a Kistler 6125 piezoelectric pressure transducer and a Kistler 5011 charge amplifier. The pressure data triggered by the 2500 pulses/rev shaft encoder were recorded against the CA and ensemble averaged over 50 cycles. The net heat release was determined from the heat release equation given by first-law analysis11 and is given by dQn g dV 1 dp p + V = g  1 du g  1 du du

ð1Þ

Geometric effects of offsetting the crankshaft and the valve timing changes Offsetting the crankshaft as depicted in Figure 1 influences the stroke length and thus the engine displacement through changes in the piston displacement as shown in Figure 2. The increase in the stroke arises from the change in the angle between the connecting rod and the cylinder liner axis according to DðuÞ = r cos u +

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi l2  ðr sin u  fÞ2

ð2Þ

where r is the crank radius, l is the connecting-rod length and f is the offset magnitude.

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Proc IMechE Part D: J Automobile Engineering 228(5) For the test engine, a 20 mm offset increases the stroke by 0.82 mm (about 1%). To maintain a fixed compression ratio with changes in the swept volume and the clearance volume, the cylinder head–liner assembly was shifted vertically along the liner axis. Shims were inserted between the crankcase and the head–liner assembly, with the required shim thickness calculated from the target compression ratio and the known crankshaft offset. When the crankshaft is offset, the crank positions at which the piston reaches its TDC and BDC are different. At the offset of 20 mm, the TDC occurs 5.5° CA later and the BDC occurs 10.2° CA later than for the zero-offset case (Figure 2 and Figure 3). Correspondingly, the compression and exhaust strokes occur over a shorter CA period and the intake and expansion strokes occur over a longer period. The 20 mm offset increased the intake stroke by 4.7° CA, or 2.6%. In order that the valve timings are matched to the changes in the piston movement produced by offsetting the crankshaft by 20 mm, the engine camshafts were rotated relative to their baseline positions. Two new orientations were used to cover the options of maintaining either the same valve overlap angle as for the zerooffset case or the same inlet and exhaust valve opening times relative to the TDC and the BDC respectively of the piston. This second case produces an increase in the valve overlap, from 18° CA to 22.7° CA. Details of the valve timings are given in Table 1.

Figure 1. Geometric representation of the piston–crank– connecting-rod assembly, showing the components of the piston displacement expression and how the expression is modified by the presence of the crankshaft offset.

Results Tests were carried out at four speed–load combinations for each of the two cam timing set-ups with the

Figure 2. Cylinder volume history for zero crank offset and 20 mm crank offset. Note that the geometric TDC occurs at + 5.5° after the crankshaft TDC with the 20 mm offset. ATDC: after top dead centre.

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Table 1. Engine specifications and valve timings. Parameter (units)

Fuel injection equipment Connecting-rod length (mm) Compression ratio Bore (mm) Stroke (mm) Displacement (cm3) BDC intake (deg CA) TDC compression (deg CA) Valve overlap (deg CA)

Value for the following crankshaft offsets and cam configurations 0 mm

20 mm

Baseline

Fixed valve overlap

Fixed valve opening

High-pressure common-rail system; maximum pressure, 1800 bar; seven-hole solenoid injector 160.0 160.0 160.0 17.5 17.5 17.5 86.0 86.0 86.0 94.6 95.4 95.4 549.5 554.3 554.3 180 190.2 190.2 360 365.5 365.5 18 18 22.7

BDC: bottom dead centre; CA crank angle; TDC: top dead centre.

Table 2. Operating conditions (the injection parameters are demanded quantities). Parameter (units)

Value for the following engine speeds and engine loads 1000 r/min

mpilot (mm3) t pilot(ms) mmain (mm3) Base start of the main injection (deg ATDC) Intake pressure (bar) Intake temperature (°C) Exhaust back pressure (bar) Fuel rail pressure (bar)

1750 r/min

Low load

High load

Low load

High load

1 1820 10.7 + 1.5 Atmospheric 29 0.10 430

1 2066 39.8 + 2.2 1.14 29 0.26 630

1 1286 12.3 + 1.1 Atmospheric 29 0.32 750

1 1487 37.0 + 1.8 1.53 29 0.83 1130

ATDC: after top dead centre.

Figure 3. Cam timing diagrams (a) for zero offset, (b) for 20 mm offset assuming that the cam timings are changed to retain the same valve overlap and (c) for 20 mm offset assuming that the cam timings are changed to retain the same opening times relative to the new TDC–BDC period. TDC: top dead centre; CA: crank angle; IVO: intake valve opening; BTDC: before top bottom dead centre; EVC: exhaust valve closing; ATDC: after top dead centre; EVO: exhaust valve opening; IVC: intake valve closing; BDC: bottom dead centre; BBDC: before bottom dead centre; ABDC: after bottom dead centre.

crankshaft offset at 20mm. The results are compared with those obtained with the standard cam timing and zero crankshaft offset. The speed–load combinations covered two fixed fuelling settings representing

a high-load condition and a low-load condition for two engine speeds of 1000 r/min and 1750 r/min (Table 2). The fuelling strategy consisted of a pilot injection and a main injection, with a fixed

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pilot-to-main spacing. During a test, the injection timing was swept through a range of 65° CA from the base timing at 2.5° CA intervals. The base timing values were those used in the standard calibration for the production version of the engine. The tests and injection timing sweeps were repeated a minimum of four times to reduce the uncertainty and day-to-day variations, and 95% confidence limits are attached to the data points in the figures described in the following.

Cylinder pressure and heat release The ensemble-averaged cylinder pressure, the net heat release and the instantaneous work P dV/du on the piston at the base-injection-timing settings are illustrated in Figures 4 and 5 for two of the speed–load points. The results at the other two speed–load points were very similar at the same loads and showed no sensitivity to the change in the engine speed, or to which of the cam settings was used. The results presented are for the fixed-valve-overlap case (Table 1). The most evident difference between the results for zero offset and an offset of 20 mm is that, for the same pressure at inlet valve closing, the cylinder pressure is higher with the offset crankshaft for both boosted and naturally aspirated conditions. This is attributed to the shorter compression process, allowing less time for heat transfer and blow-by losses. Although the instantaneous net heat release variations should reflect any changes in the heat transfer and the heat release due to combustion, the differences are only clear in the cumulative heat transfer values. For the offset crank, the cumulative totals are consistently slightly lower in the low-load condition, owing to poorer pilot combustion. This adds to the effect of increased compression work. Although the P dV/du plots in Figure 4 and Figure 5 illustrate that, while both the compression work and the expansion work increase, the former increased more. At the high load, because of the higher fuelling rates, the changes in the cumulative heat release become more important than the change in the compression work. At 1000 r/min, the heat release increased and, at 1750 r/min, the heat release was unchanged. These changes are not directly the result of offsetting the crankshaft, which may nevertheless exert an indirect influence through the gas exchange and mixture preparation processes.

Emissions and mean effective pressures

Figure 4. In-cylinder analyses for the 1750 r/min low-load base-injection-timing case: average plot from all repeats.

The results are plotted against the start of the maininjection timing, in degrees CA, for the low-load cases in Figures 6 and 7 and for the high-load cases in Figures 8 and 9. The offset crankshaft results show that the net and gross values of the indicated mean efffective pressure (IMEP) were lower for the low-load condition when the compression work increased and the cumulative heat release was lower. In the high-load condition,

CA: crank angle; RoHR: rate of heat release; ATDC: after top dead centre.

the IMEP increased at 1000 r/min and was unchanged at 1750 r/min, consistent with the trends in the cumulative heat release.

MacMillan et al.

505 by up to 2–3% for the low load. Because the injected fuel level for the zero-offset case matched that for the 20 mm offset case, these increases are associated with increases in the trapped air mass. The factors responsible for this include an increase in the swept volume of about 0.9% for the 20 mm offset and an increase of about 2.5% in the cylinder volume at intake valve closing for the same overlap value. The importance of the valve timings is indicated by the relative insensitivity of the AFR to the offset when the valve opening times are maintained and the valve overlap is higher. At 1000 r/ min for a low load, the direction of the AFR change is actually reversed. With respect to the emissions results included in Figures 6 to 9, the changes in the nitrogen oxide (NOx) emissions, the cabon monoxide (CO) emissions and the FSN were generally small. The most significant changes were measured for the 1000 r/min high-load condition at which the NOx emissions increased by up to around 15% and the CO emissions and the FSN decreased by similar proportions. This is attributable to the increased oxygen availability, promoting NOx formation on the one hand and increased oxidation of CO and soot on the other. The gross ISFC plots given in Figures 6 to 9 should directly reflect the gross IMEP values measured in a given test, because the demanded fuelling was independent of the offset. In practice, uncertainty in the delivered fuelling due to small uncontrolled variations in the experimental conditions and the fuel consumption measurements makes it unsafe to conclude more than that the offset did not produce changes in the specific fuel consumption of more than 1% in either direction.

Conclusions

Figure 5. In-cylinder analyses for the 1000 r/min high-load baseinjection-timing case: average plot from all repeats. CA: crank angle; RoHR: rate of heat release; ATDC: after top dead centre.

The results for the variations in the AFR show the effect that the offset crank and the cam profiles have on the gas exchange processes. When the overlap is maintained the same, the AFR was consistently leaner at the four speed–load points, by 4–5% for the high load and

Offsetting the crankshaft by 20 mm produced changes in the indicated performance and the emissions, which were small. Compared with the results for the baseline zero-offset case, the changes were typically no more than a few percentage points. The magnitude and direction of the changes were similar for the two cam profiles used to maintain either the valve overlap or the inlet and exhaust opening times relative to the TDC and BDC respectively of the piston. The trends are summarised in Table 3. The largest changes in the NOx emissions were consistently increases; the largest changes in the CO emissions and the FSN were consistently reductions. Small increases in the ISFC occurred for the low load and a larger reduction occurred for one high-load condition. The IMEP decreased for the low load and increased or was unchanged for the high load. Overall, the effect of the offset on the indicated performance is minor and secondary to the effect on the rubbing friction. The effects are similarly small when either the valve overlap or the valve opening time is maintained when the crankshaft is offset, although changes in the air flow characteristics appear to be as influential as the

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Figure 6. Summary responses for the 1000 r/min low-load case: averaged results with 95% confidence limits. IMEP; indicated mean effective pressure; SOI: start of injection; ATDC: after top dead centre; BMEP: brake mean effective pressure; ISFC: indicated specific fuel consumption; AFR: air-to-fuel ratio; NOx: nitrogen oxides; CO: carbon monoxide; FSN: filter smoke number.

Figure 7. Summary responses for the 1750 r/min low-load case: averaged results with 95% confidence limits. IMEP; indicated mean effective pressure; SOI: start of injection; ATDC: after top dead centre; BMEP: brake mean effective pressure; ISFC: indicated specific fuel consumption; AFR: air-to-fuel ratio; NOx: nitrogen oxides; CO: carbon monoxide; FSN: filter smoke number.

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Figure 8. Summary responses for the 1000 r/min high-load case: averaged results with 95% confidence limits. IMEP; indicated mean effective pressure; SOI: start of injection; ATDC: after top dead centre; BMEP: brake mean effective pressure; ISFC: indicated specific fuel consumption; AFR: air-to-fuel ratio; NOx: nitrogen oxides; CO: carbon monoxide; FSN: filter smoke number.

Figure 9. Summary responses for the 1750 r/min high-load case: averaged results with 95% confidence limits. IMEP; indicated mean effective pressure; SOI: start of injection; ATDC: after top dead centre; BMEP: brake mean effective pressure; ISFC: indicated specific fuel consumption; AFR: air-to-fuel ratio; NOx: nitrogen oxides; CO: carbon monoxide; FSN: filter smoke number.

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Table 3. Summary of the responses for the speed-load conditions examined relative to the zero-offset baseline. Arrows point up for an increase and down for a decrease. Thin arrows indicate marginal changes. Response for the following engine loads, speeds and cam profiles 1000 r/min Low load

1750 r/min Low load

1000 r/min High load

1750 r/min High load

Same overlap

Same overlap

Same overlap

Same overlap

Same valve opening

IMEP

—

—

BMEP

—

—

—

—

—

—

—

—

—

—

Parameter

Same valve opening

Same valve opening

—

AFR NOx emissions

—

—

CO emissions

—

—

Smoke emissions (FSN) —

—

ISFC

Same valve opening

—

—

— —

—

IMEP; indicated mean effective pressure; BMEP: brake mean effective pressure; AFR: air-to-fuel ratio; NOx: nitrogen oxides; CO: carbon monoxide; FSN: filter smoke number; ISFC: indicated specific fuel consumption.

thermodynamic changes when offsetting the crankshaft. Separation of the relative contributions of each of these changes is difficult and highlights the importance of cam timing selection. There is negligible change in the thermodynamic performance and the emissions of a light-duty diesel engine when employing a crankshaft offset of 20 mm. Engine designs which feature offsets of this magnitude to reduce friction, improve NVH or secure other benefits should not suffer penalties in combustion or emissions. Acknowledgement The authors are pleased to acknowledge the Ford Motor Company for permission to publish the results. Funding This work was supported by the Ford Motor Company. Declaration of conflicting interest

4. Sano S, Kamiyama E and Ueda T. Improvement of thermal efficiency by offsetting the crankshaft center to the cylinder bore center. JSAE Rev 1997; 18(2): 206 (JSAE paper 9638770, 1996). 5. Wakabayashi R, Takiguchi M, Shimada T et al. The effects of crank ratio and crankshaft offset on piston friction losses. SAE paper 2003-01-0983, 2003. 6. Nakayama K, Tamaki S, Miki H and Takiguchi M. The effect of crankshaft offset on piston friction force in a gasoline engine. SAE paper 2000-01-0922, 2000. 7. Shin S, Cusenza A and Shi F. Offset crankshaft effects on SI engine combustion and friction performance. SAE paper 2004-01-0606, 2004. 8. Guzzomi AL, Hesterman DC and Stone BJ. Variable inertia effects of an engine including piston friction and a crank or gudgeon pin offset. Proc IMechE Part D: J Automobile Engineering 2008; 222(3): 397–414. 9. Cho M-R, Kim J-S, Oh D-Y and Han D-C. The effects of crankshaft offset on the engine friction. Int J Veh Des 2003; 31(2): 187–201. 10. Jibben JJ. Analysis of an extended stroke, (offset crankshaft), engine. SAE paper 2006-01-0014, 2006. 11. Heywood JB. Internal combustion engine fundamentals. New York: McGraw-Hill, 1988

The authors declare that there is no conflict of interest. References 1. Friedfeldt R, Zenner T, Ernst R and Fraser A. Threecylinder gasoline engine with direct injection. ATZ Autotechnol 2012; 12(2): 34–40. 2. Lee S, Gu Y, Kim T and Han J. The new Hyundai-Kia 1.0l three cylinder gasoline engine. MTZ Worldwide eMag 2011; 72(7–8); 10–17. 3. Cho M-R, Oh, D-Y Moon T-S and Han D-C. Theoretical evaluation of the effects of crank offset on the reduction of engine friction. Proc IMechE Part D: J Automobile Engineering 2003; 217(10): 891–898.

Appendix 1 Notation D f l p Q r V g

displacement crankshaft offset connecting-rod length pressure heat crank radius volume ratio of specific heats

MacMillan et al. u t

crank angle dwell time

Abbreviations ABDC AFR ATDC BBDC BDC BMEP BTDC CA CO EVC EVO

after bottom dead centre air-to-fuel ratio after top dead centre before bottom dead centre bottom dead centre brake mean effective pressure before top bottom dead centre crank angle carbon monoxide exhaust valve closing exhaust valve opening

509 FSN IMEP ISFC IVC IVO NOx SOI TDC

filter smoke number indicated mean effective pressure indicated specific fuel consumption intake valve closing intake valve opening nitrogen oxides start of injection top dead centre

Subscripts g main n pilot

gross main injection net pilot injection