Variable Compression Ratio (VCR) Assessment to Enable Higher Efficiency in Gasoline Engines

DOE FY 2012 Advanced Combustion Technologies Annual Report

Variable Compression Ratio (VCR) Assessment to Enable Higher Efficiency in Gasoline Engines

ORNL Project Manager:

Norberto Domingo

Oak Ridge National Laboratory (ORNL)

2360 Cherahala Boulevard

Knoxville, TN 37932

865-946-1229

Subcontractor:

Charles Mendler

ENVERA

425 North Alfred Street

Los Angeles, California 90048

Phone: (415) 381-0560

Email:

DOE Technology Development Manager:

Roland Gravel(202) 586-9263

email:

Overall Objectives

• Under subcontract with Envera LLC, design, prototype, and deliver to ORNL one variable compression ratio (VCR) gasoline direct injection engine (GDI) with combineddirect fuel injection and port fuel injection capabilities. Conduct high-load dynamometer testing prior to delivery ORNL to validate functionality of all mechanical systems.
• Set up VCR engine at ORNL and quantify the fuel economy benefit of variable compression ratio (VCR) engine technology to a modern direct-injection gasoline engine, and the impact on emissions.
• Map VCR engine performance and emissions over multiple control variables, including compressionratio, spark timing, and cam phasing.
• Utilize experimental engine maps as inputs to drive cycle simulation software to estimate the real-world fuel economy and emissions impact.

Fiscal Year (FY) 2012Objectives

• Continue work on fabrication of prototype VCR research engine. This includes machining of engine bedplate, engine crankshaft cradle, and fabrication of custom crankshaft and other VCR actuation components.
• Deliver all engine components to Automotive Specialist in Concord, NC., for engine build and demonstration testing.
• Conduct a post test inspection of key engine components by Automotive Specialist and reassemble engine for delivery to ORNL.

Accomplishments

• The Envera VCR engine has been built, and is now being prepared for dynamometer testing. Testing of the engine at Automotive Specialists is scheduled for the week of December 10th, 2012.

• The engine was first preassembled by Envera in Los Angeles, California. Figures 1 and 2 shows the VCR engine preassembly. The engine was then disassembled and shipped to Automotive Specialists.

• Automotive specialists then reassembled the engine in North Carolina. Automotive Specialists finalized key mechanical dimensions such as cylinder honing, thrust bearing clearances and cam timing checks. General mechanical functionality was inspected during assembly. Figures 3 and 4 show the Envera VCR engine in the Automotive Specialists engine build room.

Figure 1: Envera VCR Engine Preassembly

Figure 2: VCR Crankshaft cradle and crankshaft preassembly

Figure 3: Front view of the VCR engine at Automotive Specialists

Figure 4: Rear view of the VCR engine at Automotive Specialists

Future Directions

• Conduct demonstration testing of VCR engine including measurements of the engine performance, VCR actuator mechanism response time and fuel consumption at Automotive Specialists in Concord, NC.
• Conduct a post test inspection of key engine components by Automotive Specialist and reassemble engine for delivery to ORNL.
• Install engine at ORNL dynamometer test facility and conduct experiments to investigate the effect of variable compression ratio(CR) on engine efficiency at the maximum brake torque (MBT) spark timing over the engine map with a variable CR research engine. Engine experiments will combine parametric sweeps of spark timing, CR, and cam phasing to determine the optimal efficiency at each engine speed/load operating condition. Emissions from the engine will also be measured before and after a 3-way catalyst.

Introduction

Basic thermodynamics dictate that the efficiency of internal combustion engines is proportional to compression ratio (CR). However, CR of modern gasoline engines is relatively low from a thermodynamic standpoint, in the range of about 8.5 to 12.5, because of practical constraints at higher CR such as engine knock, increased friction work, and increased heat transfer. It is only the knock-prone conditions (low speed, high load) that constrain the engine CR. The efficiency of most part-load engine conditions can be increased by raising the CR, and it is these part-load conditions that have the most direct impact the real-world fuel economy.

Figure 5 illustrates the potential impact of increased part-load efficiency on fuel economy. Figure 5 (a) shows a contour map of engine efficiency as functions of engine speed and load for a Saab Biopower vehicle, and is qualitatively representative of modern engines. Peak engine efficiency is approximately 32%. Figure 5 (b) shows the vehicle speed as a function of time for an FTP driving cycle, used in determining EPA “City” fuel economy, which contains numerous decelerations, stops, and accelerations. In Figure 5 (c), data points from the FTP cycle are superimposed on the map of engine efficiency. Nearly all of the driving cycle takes place under low efficiency part-load engine conditions, conditions where higher compression could increase engine efficiency.

(a) (b) (c)

Figure 5. (a) Engine efficiency contour plot for the Saab Biopower, (b) Vehicle speed trace for the FTP driving cycle, and (c) Operating points from the FTP driving cycle superimposed on the engine efficiency contour plot. Data collected at the ORNL chassis dynamometer laboratory in 2007.

There are currently several ongoing investigations aimed at increasing efficiency, at least in part, by utilizing high mechanical CR and reducing the effective CR with late intake valve closing[1,2]; a technique that is particularly attractive for optimizing E85 engines. However, reducing the effective CR in this manner actually decreases the maximum torque at all engine speeds because of reduced volumetric efficiency, which is a direct result of late intake valve closing. An earlier investigation to increase part-load efficiency was made with a predecessor of the research engine proposed here [3].The earlier investigation demonstrated proof of principle and showed promise, but did not include a comprehensive study on the effects of variable CR engine emissions and is no longer representative of modern engine technology.

Engine downsizing is viewed by US and foreign automobile manufacturers as one of the best options for improving fuel economy. While this strategy has already demonstrated a degree of success, downsizing and fuel economy gains are currently limited. With new variable compression ratio technology however, the degree of engine downsizing and fuel economy improvement can be greatly increased. A small variable compression ratio (VCR) engine has the potential to return significantly higher vehicle fuel economy while also providing high power.

To meet torque and power requirements, a smaller engine needs to do more work per stroke. This is typically accomplished by boosting the incoming charge with either a turbo or supercharger so that more energy is present in the cylinder per stroke to do the work. With current production engines the degree of engine boosting (which correlates to downsizing) is limited by engine knock at high boost levels. Engine knock or detonation can be prevented by lowering the compression ratio and using premium octane fuel, but as stated earlier, lowering compression ratio reduces engine efficiency and using premium fuel increases customer cost.

VCR technology eliminates the limitation of engine knock at high load levels by reducing compression ratio to ~8.5:1 (or whatever level is appropriate) when high boost pressures are needed and regular grade fuel is used. By reducing the compression ratio during high load demand periods there is increased volume in the cylinder at top dead center (TDC) which allows more charge (or energy) to enter the cylinder without increasing the peak pressure. Cylinder pressure is thus kept below the level at which the engine would begin to knock. When loads on the engine are low the compression ratio can be raised (to as much as 18:1) providing high engine efficiency. It is important to recognize that for a well designed VCR engine cylinder pressure does not need to be higher than found in current production turbocharged engines. As such, there is no need for a stronger crankcase, bearings and other load bearing parts within the VCR engine.

Under the current program, Envera is delivering a prototype gasoline direct-injection VCR engine to ORNL. In the proposed study, ORNL will provide anupdate of the potential benefit of variable mechanical CR on efficiency and emissions using Envera’s state of the art VCR engine technology.

Fuel Efficiency Comparison

The fuel efficiency benefit of downsizing from a naturally aspirated V8 engine to a turbocharged V6 engine was assessed by the US Environmental Protection Agency (A Study of Potential Effectiveness of Carbon Dioxide Reducing Vehicle Technologies, Revised Final Report, June 2008, US EPA)[4] and Ricardo(SAE Paper No. 2007-01-1410)[5]. In Figure 6, engine efficiency projections for an in-line 4-cylinder Envera VCR engine are added to the earlier EPA/Ricardo comparison. Figure 6 includes brake specific fuel consumption (BSFC) curves for a gasoline direct injection turbocharged 3.6L V6 engine (DI Boost) and a naturally aspirated engine 5.7L V8 engine (V8) as reported by EPA. Efficiency projections for the Envera VCR engine (dashed black line) have been added to the graph. All three engine BSFC curves correspond to an engine speed of 2000 rpm. The Envera VCR engine will operate according to the Atkinson Cycle at light load, with engine calibration settings similar to (or better than) that of the 2010 Toyota Prius. The solid portion of the “--- VCR BOOST” curve is drawn from engine efficiency data presented by Toyota in SAE paper 2009-01-1061[6]. The data has been scaled from an engine having a displacement of 1.8 L (the stock 2010 Toyota Prius) to 2.2 L so that the peak torque of the Envera VCR engine matches the peak torque of the V8 engine at 2000 rpm, which is 445 Nm (328 ft-lb). In this comparison the Envera VCR engine employs turbo charging in order to provide V8-like power and torque. VCR and variable valve actuation enable the Envera engine to operate according to the high-efficiency Atkinson cycle at light loads and according to the Otto-cycle with aggressive turbo charging at high loads. Data for the three engines is shown in Table 1. At 100 Nm and 2000 rpm, all three engines produce about 28 horsepower. The V6-Turbo DI engine is expected to consume 19.6 percent more fuel than the I4-turbocharged VCR engine at 100 Nm. The V8 engine is expected to consume 43 percent more fuel than the I4-turbocharged engine at 100 Nm. At 50 Nm (14 hp) the V8 engine is expected to consume 67 percent more fuel than the I4-turbocharged engine. The VCR engine in this comparison has port fuel injection. Even larger gains in fuel efficiency may be attainable for gasoline direct-injection VCR engines. In general, VCR technology enables significantly larger fuel economy gains to be achieved than can be accomplished with fixed compression ratio engines.

TABLE 1

TypeV8V6-Turbo DII4-Turbocharged VCR

Displacement5.7L3.56L2.2L

Power298 kW283 kW271 kW

At 2000 RPM

Peak Torque445 Nm560 Nm445 Nm

Peak BMEP9.8 bar19.7 bar25.4 bar

Fuel efficiency

@ 100 Nm328 g/kWh276 g/kWh230 g/kWh

@ 50 Nm500 g/kWh362 g/kWh300 g/kWh

Approach

We propose to investigate the effect of variable compression ratio on engine efficiency at the maximum brake torque (MBT) spark timing over the engine map with a variable compression ratio research engine designed and built by Envera LLC. This state of the art engine will combine a production cylinder head utilizing direct fuel injection, cam phasers, and a custom engine block containing the VCR mechanism. Specifications for the engine are shown in Table 2.

Engine experiments will combine parametric sweeps of spark timing, CR, and cam phasing to determine the optimal efficiency at each engine speed/load operating condition. Emissions from the engine will also be measured before and after a 3-way catalyst.

The experimental data of engine performance and emissions will be used to generate composite engine maps of engine efficiency and emissions (i.e. at best efficiency, constant CR, lowest emissions, etc). The engine maps will be used as inputs for computer simulations of vehicle drive cycles using the PSAT software to determine the real-world impact of variable CR on fuel economy and emissions.

Table 2. ORNL Envera GDI- VCR Engine Specifications

CylindersInline 4-cylinder

Displacement1.886L

Bore / Stroke81.0 / 91.5 mm

Compression ratioVariable

Maximum18:1

Minimum8.5:1

Crankcase materialA356 aluminum

CoolingElectric water pump

Valve train16-valve DOHC

Phase shiftersIntake and exhaust camshafts

Fuel deliveryGasoline direct injection

AspirationNaturally aspirated as delivered (turbocharge upgrade capable)

Results

The Envera VCR engine has been built and is now being prepared for dynamometer testing. Assembly checks on key mechanical dimensions such as cylinder honing, thrust bearing clearances and cam timing were conducted by Automotive Specialists. Testing of the engine at Automotive Specialists is scheduled for the week of December 10th, 2012.

Conclusions

VCR technology enables significantly larger fuel economy gains to be achieved than can be accomplished with fixed compression ratio engines. Under the current program, Envera is delivering a prototype gasoline direct-injection VCR engine to ORNL. ORNL will then conduct dynamometer tests to independently assess engine efficiency values, and use the engine for efficiency research and benchmarking.

References

1. Confer, K., “E85 Optimized Engine through Boosting, Spray Optimized DIG, VCR, and Variable Valvetrain.” Presented at 2008 DOE Vehicle Technology Merit Review,
2. Yilmaz, H., “DOE Merit Review – Flex Fuel Vehicle Systems.” Presented at 2008 DOE Vehicle Technology Merit Review,
3. Mendler, C. and R. Gravel, “Variable Compression Ratio Engine.” Society of Automotive Engineers, 2002, Technical Paper 2002-01-1940.
4. A study of Potential Effectiveness of Carbon Dioxide Reducing Vehicle Technologies, Revised Final Report, June 2008, US EPA.
5. Christie, M. et. al Ricardo Ltd.: DI Boost: Application of a High Performance Gasoline Direct Injection Concept, SAE Paper no. 2007-01-1410, Pub. SAE 2007.

1. Kawamoto, N. et al. Toyota Motor Corporation: Development of New 1.8-Liter Engine for Hybrid Vehicles, SAE Paper 2009-01-1061, Pub SAE 2009.

Acronyms

BSFCBrake specific fuel consumption\

CRCompression ratio

DOEUnited States Department of Energy

DOHCDouble overhead camshafts

EPAUnited States Environmental Protection Agency

GDIGasoline direct injection

NETLNational Energy Technology Laboratory

ORNLOak Ridge National Laboratory

TDCTop dead center

VCRVariable Compression Ratio