Optimizing Hybrids for Cost and Efficiency

AVL engineers detail a modular engine program suitable for all forms of hybridization.

April 1, 2021

Figure 1: Diversity of powertrain electrification by vehicle use.

The electrification of the powertrain is a prerequisite to meet future fuel consumption limits, while the internal combustion engine (ICE) will remain a key element of most production-volume-relevant powertrain concepts.

The range presented by AVL will include parallel hybrids, 48V- or high-voltage mild or full hybrids, up to serial hybrids. In the first configurations, the ICE is the main propulsion, requiring the whole engine speed and load range including the transient operation. In serial hybrid applications, the vehicle is generally electrically driven; the ICE provides power to drive the generator, either exclusively or supporting a battery charging concept. When the ICE is not mechanically coupled to the drivetrain, a reduction of the operating range and thus a partial simplification of the ICE is achievable.

The diversity of possible powertrain variants presents a challenge for vehicle OEMs and powertrain or engine manufacturers, implying parallel development of many different technologies as well as increased manufacturing complexity.

Figure 1 illustrates the large variety of electrification topologies currently in use, including battery- and fuel-cell electric as well as electrified ICE based powertrains in various degrees of hybridization. The choice of topology for each application is dependent on the use case. For the extreme cases (like a heavy-duty truck or small city car) the choice is relatively clear; for a mid-sized SUV, located in the middle of this chart, many different configurations could be considered.

The present study was based on a D-segment SUV application. This represents both a technical challenge in terms of the legislative boundaries of emission and fuel-consumption targets, and a key market segment responsible for a large share of fleet fuel consumption. As the fleet targets are reduced, the share of vehicles with higher degrees of electrification must be increased.

Powertrain and ICE power range

A boundary condition for electrified powertrain variants is the achievement of at least the same vehicle performance compared to the conventional versions, but at lower fuel consumption. Typically, the demanded power for this segment is in the range from 140 kW up to 240 kW for the main vehicle variants, though specific top-performance versions may be higher.

Figure 2: Degrees of electrification: from ICE to fuel cell.

The increasing electrification range (Figure 2) will include 48V-P0 micro-hybrids, more or less replacing the 12V systems as the new standard, as well as parallel hybrids in 48V-P2 or P4 configurations, high-voltage full hybrids both without and increasingly with plug-in functionality and finally, serial hybrids.

The first level of hybrid system (P0 position) uses a 48V belt starter-generator (BSG) system, substituting the more powerful alternator for the 12V machine, in the same belt-drive layout. The 48V supply is a pre-requisite for the e-supercharger, respectively e-turbo-charger, used in the higher-performance engine variants, as well as a limited electric driving range. The electric power is around 12kW.

Figure 3: 48V electrification locations: P0, P2 and Hybrid Transmission.

The next level in powertrain electrification is a P2 configuration, remaining at the 48V level and delivering around 20kW electric power. The arrangement as shown in Figure 3 with the e-machine packaged parallel to the gearbox allows the integration in transverse installations, usually very sensitive regarding overall powertrain length.

The configurations with further increased electrical performance support extended recuperation capability, torque assist, load-point shifting of the ICE and full electric drive. Integrating the e-motor into the transmission architecture allows integration of electrical and mechanical functions in P2 or P2.5 architectures (Figure 3, right). A modular hybrid transmission family (“Direct Hybrid Transmission”), allowing the integration of 48V as well as high-voltage e-motors is the basis for the 400V variants. The power of the electric motor is increased to around 120kW.

Engine tech packages for high combustion and performance

Figure 4: Tradeoff between engine brake thermal efficiency and specific power.

Future emission requirements including more highly-loaded emissions certification cycles can be achieved with an aftertreatment package consisting of a 3-way catalyst for gaseous emissions and a particle filter to control particulates in all operating conditions. Robust compliance with real driving emissions limits demands 3-way catalytic conversion and therefore stoichiometric combustion in the whole engine map. Figure 4 illustrates the limitations of different measures to support this, in terms of fuel consumption and achievable power ratings.

Measures start with active exhaust gas cooling in the form of an integrated exhaust manifold, already implemented on many turbocharged gasoline engines. Water injection can extend the stoichiometric operation up to high power from 120 kW/L to above 170 kW/L and will be the choice for concepts prioritizing power and emissions over fuel consumption.

On the other hand, Miller-cycle valve timing with high geometric compression ratio and extended expansion offers a route to improve brake thermal efficiency while limiting the achievable power density. Power density can be maintained by increasing boost pressure to compensate for the short valve-opening duration.

Variable compression ratio (VCR) is an enabler to maximize the potential of Millerization for higher-performance variants. The implementation as a 2-step system using a switchable connecting rod design with hydraulic actuation has been described in detail: https://www.sae.org/publications/technical-papers/content/2017-01-0634/ . As shown, the 2-step VCR with a compression ratio range of 10 at part load to 15 at full load gives most of the benefit expected from a continuously variable system. In combination with variable intake valve lift, VCS proves to be very effective both in view of stochiometric range and minimum fuel consumption.

The efficiency targets for next-generation combustion engines will be significantly tightened. For the next step towards 50% brake thermal efficiency, a new generation of technology packages is in development, including pre-chamber spark ignition and ultra-high-pressure injection. The aim is to allow high compression ratio in the whole map while suppressing knock. One approach to achieve that is a reduction of the charge-air temperature close to ambient temperature, by use of a two-stage e-supercharger with dual intercooling.

A modular charging concept including electrified components such as e-superchargers and e-turbos enables optimized use of electric energy in the hybrid powertrain. For acceleration utilizing the thermodynamic amplification (electric energy is utilized to compress air and/or spin up the TC) rather than direct electric power, the battery can be reduced in size without loss of vehicle performance.

With an electrified powertrain supplementing engine torque at low engine speed, the exhaust-gas turbocharger systems can be rematched with focus on the high-speed area. This extends the potential application of each charging technology towards higher specific power, leading to a more cost-effective overall package.

Table 1: Power targets of the modular ICE variants with and without variable compression-ratio technology.

Table 1 shows variants in each power class based on current-production TGDI technology base and the potential for efficiency improvement with application of the above technologies. By applying the above-mentioned technology elements, the full range of ICE power can be covered by an engine family based on a turbocharged in-line 4-cylinder with about 2.0L displacement and specific power ratings from 70 to 120kW/L.

Modular ICE technology packages

An ICE engine family with modular components is proposed to avoid developing a dedicated base engine for each application. As well as a significant cost advantage due to higher component production volumes, this approach gives high flexibility considering uncertain future volume distributions. This approach has been proposed before and implemented by several OEMs in production. The concept is now adapted and extended to include variation based on the degree of electrification.

The basic engine architecture has been laid out from the outset to minimize mechanical losses of the base engine:

long stroke (Stroke/Bore > 1.2)
crankshaft offset 12~15% of stroke
long conrod (L/r > 3.3)
minimization of bore distortion by structural
optimization and shape honing
valvetrain with low friction (RFF+HLA)
minimized diameter of main bearings
demand-controlled piston cooling jets
split cooling (separate cooling circuits for cylinder head and block)
chain-driven oil pump, pressure and volume flow is demand-controlled

Figure 5: Modular engine friction-reduction measures.

In addition to these concept features, further measures that could be incorporated with minimal changes to the production and assembly lines, even for existing production facilities, including:

electronically controlled thermostat/coolant temperature-management module
second-order mass balancing shafts with roller bearings (Figure 5)
friction reducing coatings (piston rings, piston pins)
camshaft roller bearings (1st bearing)
switchable high performance water pump (low-friction seal, efficiency-optimized impeller)

Crankcase and cranktrain

Figure 6: Components of AVL’s variable-compression system.

The crankcase structure must address both mechanical and thermal loads considering gasoline engines with high power density and hence high thermal load as well as peak firing pressure requirements of 150 bar (2,176 psi) for future knock-free combustion. Due to the significant weight advantage, aluminum is proposed with and open deck structure.

For engine variants with variable compression ratio, the conventional connecting rod is replaced by the VCS variable conrod. In this case, adaptation of the crankshaft (conrod journal width) is required as well as the addition of the hydraulic control system including electric booster oil pump and pressure accumulator as shown in Figure 6.

Cylinder head, valvetrain and cooling

Figure 7: Modular cylinder head and valve train assembly.

A modular architecture with compact base cylinder head and separate cam carrier module permits the standardization of the main interfaces such as cylinder head height or camshaft positions. This approach, while also saving costs and weight, provides flexibility for the application of different variability of the valvetrain systems (Figure 7) without modifying the cylinder head itself. The base cylinder head with combustion chamber, gas exchange ports and cooling jacket has a cooled, integrated exhaust gas manifold with compact routing.

Cylinder head cooling for turbocharged engines with higher power density requires special attention. A top-down cooling concept developed specifically for 4-valve cylinder heads features two cooling jackets in the cylinder head, joined by efficient cooling jets between the exhaust valves of each cylinder.

The base variants of the represented engine family possess switchable water pumps with map-controlled thermostat. A fully variable thermo-management module allowing zero flow at low, part-load operation is mandatory for future high efficiency concepts.

For electrification variants with 48V, an electric water pump becomes an attractive alternative, providing true on-demand flow and significantly reducing power demand for pump actuation. applied at all hybrid versions. For the hybrid variants the cooling system complexity further increases with cooling of the electrical components.

A pressure- and volume- flow-controlled oil pump is considered as a baseline technology for all engine variants. Piston cooling jets are one of the most significant consumers in terms of volume flow rate. Demand-based control of the cooling significantly reduces oil flow rate and associated parasitic losses in the low-load range. The integrated, controlled piston cooling that controls the oil flow rate to the jets according to the actual operating point of the engine is therefore a standard feature of all variants.

Technology features of performance variants

Starting with the baseline 48V-P0, three different performance levels are proposed, with respective 80-, 100- and 120-kWL outputs. The base variant with 80 kW/L has a 350-bar (5,076-psi) fuel injection system and variable-geometry turbine (VGT). An external EGR-system is part of the fuel efficiency and emission package.

The valvetrain for all variants includes continuous variable valve timing at intake and exhaust. For the 80kW/L variant a 2-step cam switching system at the intake is optional in combination with VGT. For the higher ratings, 2-step valve lift is required, allowing switching between part-load Miller cam and full-load cam.

Figure 8: The 100 kW/L performance variant of the modular engine includes variable compression ratio and variable- geometry turbocharging.

For the 100kW/L engine variant, a variable turbo geometry is the base variant as well. An e-VGT is an option for this rating. The base features and the two considered combinations of additional features are shown in Figure 8.

Electrical support of the charging system is a mandatory feature for a high efficiency powertrain layout at these highest ratings, considering transient operation.

48V MHEV beltless engine

The next level in powertrain electrification is a P2-configuration, remaining at the 48V level but with up to 20kW electrical power. As the powertrain configuration offers increased electrical performance, replacement of mechanical auxiliary functions by electrical solutions becomes possible.

The simplification of the ICE auxiliary drives as well as the enhanced demand control lead to electrification of power steering, vacuum pump, coolant pump and A/C compressor. A beltless ICE without auxiliary drives would be the logical step for hybrid application.

With the electrical performance of the 48V-P2 configuration the full load curve of the ICE does not need to provide low-end torque. Thus, the characteristics of the charging unit can be tailored to operate in the range of highest charger efficiency at rated power.

The ICE for high-voltage HEV and PHEV

Combining ICE powertrains with high voltage systems provides simplification potential on the ICE by limiting its operation range, reducing the ICE power rating even more than a 48V system. For example, in combination with an e-motor delivering up to 80kW at 400V, the ICE power rating can be reduced to 160kW.

Since the e-motor delivers high torque at low speed, a quite extreme reduction of low-end torque from the ICE up to mid engine speeds allows simplification of the boosting system to a wastegate TC, removal of the VVL system, and potentially also VVT system.

The ideal ICE for a serial hybrid or range extender

The largest step in ICE simplification can be achieved with serial-hybrid or range-extender configurations.

As the ICE is not mechanically coupled to the drive-train, a reduction of the operation range to preferred load and speed ranges with high efficiency is feasible and transient as well as idle operation is irrelevant. For vehicle operation outside the city mode, direct connection of the ICE to the drive train may be advantageous, limited to higher vehicle speeds in highway operation.

Maximum vehicle performance in high-voltage hybrid applications, in parallel mode with ICE connected to the drivetrain, is achieved with the combustion engine and e-motor combined.

So far, even with moderate ICE power, the acceleration and performance of hybrid versions is significantly beyond the conventional variants. For these applications, the question is whether a naturally-aspirated Atkinson engine, or a smaller turbocharged engine is the better option. The NA engine shows its best BSFC in a rather narrow area of the map, whereas a TC-engine achieves a wider sweet spot.

Efficiency versus cost

Figure 9: Relative powertrain costs for various technology packages.

The trade-off between add-on cost and CO2 savings on one side and robustness regarding RDE-requirements on the other side is an essential factor for the final selection of the optimum technology packages. In Figure 9, the cost for different powertrain architectures and technology packages leading to comparable system power levels are visualized. Starting point for each configuration is an EU7 emission-compliant (i.e., full map in stoichiometric operation) base engine together with a conforming aftertreatment system (again for expected EU7 targets). These two cost blocks together represent the 100% line in the diagram.

Moving from 48V to 400V, the main cost driver is the larger battery required for plug-in hybrids to increase the electric-driving range.

Conclusions

A modular engine family architecture with common machining and assembly concepts for the main components provides a basis to integrate different technology modules for different applications. By adapting charging and valvetrain systems, the full-load curve shape and fuel consumption maps can be tailored to best match the specific powertrain configuration, from pure ICE through mild- to full-hybrid applications.

This modular technology “component box” is a cost-effective and flexible way to cope with future fuel-consumption and emission limits, in particular when considering an uncertain distribution of variants in future vehicle platforms.

Balancing overall powertrain complexity with tailored technology modules on the ICE dependent on the degree of electrification allows significant cost reduction up to 40% for highly electrified variants compared to the ICE-only baseline. It at least partially compensates the cost penalty of the additional electric components.

This is a condensed version of SAE Technical Paper 2020-01-0839, “A modular gasoline engine family for hybrid powertrains: Balancing cost and efficiency.” It can be ordered or downloaded from SAE International at www.sae.org .