High tech or low cost?

FEV investigates the optimal si engine for hybrid powertrains

1. June 2017 | Engineering Service

The whole study which also includes evaluation for power split hybrid powertrains is available as a download at:



Recent market studies performed by FEV for Europe, the USA and China predict a shift toward environmentally sustainable drive systems. The main factor in this trend is the electrification of the drivetrain. Although the majority of all vehicles sold in Europe (75 to 85%) will still have a combustion engine in 2030, a high proportion of these combustion engines will be operated with hybrid powertrains. How can gasoline engines be designed to achieve low CO2 emissions in different hybrid topologies (as mild hybrid, plug-in hybrid, etc.) at optimal cost? FEV addressed this question in a broad study based on a D-segment vehicle. The reference powertrain was a conventional powertrain with a 2.0 liter gasoline engine (135 kW, TC DI with a 2-stage variable valve train and Miller cycle), a 7-speed dual clutch transmission and a 12V electrical system.

Architecture of Hybrid Powertrains

Hybrid drives are, by definition, combinations of various drives in the same powertrain. Therefore, there is a correspondingly high number of possible constellations. In this broad study, the FEV experts focused on hybrid combinations with gasoline engines and topologies with particularly high market shares or particularly high market prospects. These are:

  • Mild hybrid with 48V belt starter generator (BSG)
  • Mild hybrid with integrated 48V starter generator (ISG)
  • Plug-in hybrid (PHEV)

Cost-Benefit Comparison

A comprehensive technology matrix was examined within the framework of FEV’s study. This matrix includes turbocharged and naturally aspirated engine concepts with displacements from 1 to 3 liters. In addition, the selection focuses on technologies that are already in series production or show a high degree of maturity, indicated by a series production launch within the next 3 years. All engines comply with current and future emissions legislation such as EU6d and CN6b. They are operated stoichiometrically (λ= 1) throughout the engine map, and they are equipped with particulate filters.

With the aim of identifying the optimal combustion engine for hybrid drives, these technologies have been evaluated regarding their costs and CO2 emissions. The CO2 emissions are mean values from WLTP-L and -H. The cost scenarios relate to 2025 and a production volume of 200,000 drives/year. In addition, emission in RDE were tested and evaluated.
The result: Turbocharged 3-cylinder engines as well as naturally aspirated 4-cylinder engines with larger displacements are both suited to achieve low CO2 emissions. Rather simply designed “On/Off” technology packages are also suited for the scaling of the performance requirements of the combustion engine for various hybrid drives. The conversion to a naturally aspirated 3.0 liter 6-cylinder engine with port fuel injection (PFI), Atkinson cycle and cooled EGR neither appears favorable from the cost perspective nor from that of CO2 emissions.


Engine Variants for Mild Hybrids with 48V BSG

The electrification of the reference powertrain with 48V BSG without changes to the combustion engine costs 740 € and reduces the CO2 emissions by 8.6 g/km which means costs of 86 €/g-CO2. Starting with this drive, several technology combinations optimal for hybrids were identified. They arrived at the following conclusions for the D-segment vehicle evaluated in the WLTP cycle: Naturally aspirated 4-cylinder engines and turbocharged 3-cylinder engines are particularly well suited for use in a mild hybrid with a 48V belt-driven starter generator. Naturally aspirated engines require 2.5 liters displacement, variable valve timing on the inlet and outlet sides (dual VVT), direct fuel injection (DI) and a variable intake system to meet performance demands. The costs of the BSG drive train can be reduced to 490 € by conversion to a naturally aspirated engine with a simultaneous increase of the CO2 emission reduction to 10.4 g/km (47 €/g-CO2 is achieved). With the addition of cylinder deactivation (CDA), the costs increase again to 560 €; however the CO2 emission reduction is disproportionately increased to 12.1 g/km (46 €/g-CO2 is achieved).

With the simplification of the combustion engine to a 3-cylinder engine and the elimination of the inlet valve lift variability, the Miller cycle has to be omitted to continuously meet performance requirements. Therefore, the powertrain has a lower CO2 emission reduction of 8.9 g/km, but only costs 460 € (52 €/g-CO2 is achieved). If inlet valve lift variability and the Miller cycle are retained for the 3-cylinder engine, the performance requirement can also be met via a high temperature-proof turbine with variable geometry (950 °C VTG). A CO2 emission reduction of 13.1 g/km is therefore achieved at 46 €/g-CO2. The technology package inlet valve variability, Miller cycle and 2-stage charging system can also be replaced at a favorable cost and CO2 emissions level by a 2-stage VCR system (44 €/g-CO2).

Water injection can also serve as a substitute technology and allows a further downsizing to 1.3 liter with its strong knock-suppressing and simultaneously performance-enhancing effect. This variant with direct water injection does not yet have the same production maturity as VCR, but will yield good ratios for costs and CO2 emissions for the 2025 horizon in hybrid drives, too (33 €/g-CO2 is achieved).

Engine Variants for Mild Hybrid with 48V ISG

Expanding the hybrid functionalities by repositioning the electric engine from a P0 to a P2 layout increases the cost of powertrain electrification to 990 € but leads to a CO2 emission reduction of 19.4 g/km compared to the reference powertrain (51 €/g-CO2). The analysis shows that the technology package evaluation of a BSG mild hybrid is largely transferable to an ISG mild hybrid powertrain. Naturally aspirated 4-cylinder engines and turbocharged 3-cylinder engines are suitable for cost/CO2-optimized use in drives equipped with ISG.


Graphics and chart - SI hybrid engines

Evaluation of technology packages of gasoline engines for mild hybrids with 48V belt-driven starter generator (BSG mild hybrid) relative to the reference powertrain with regards to the ratio of costs and CO2 emissions


Graphics and chart - SI hybrid engines

Gasoline engine technology packages ratio of cost and CO2 emissions for mild hybrids with integrated 48V starter generator (ISG mild hybrid) relative to the reference powertrain

Engine Variants for PHEV

The influence of the combustion engine technology package on achievable CO2 emissions reduction is small for the plug-in hybrid drive concept. The design is characterized by compliance with the performance requirements at optimal cost. Small, simplified turbocharged engines with knock-reducing technologies like Miller valve timings or VCR and larger, naturally aspirated engines like a 2.5 liter 4-cylinder with PFI, Atkinson cycles and cooled exhaust gas recirculation (cEGR) meet this requirement.

Assessment Under Real Driving Conditions

In the WLTP cycle, the influence of the combustion engine on the CO2 emissions reduction decreases with an increasing degree of electrification. At the same time, the increasing influence of the e-motor allows for the simplification of combustion engine technologies, thus achieving cost benefits.

The influence of the combustion engine increases under real driving conditions (RDE) compared to the WLTP cycle significantly. This is due to higher loads and a smaller pro-portion of electric driving. Turbo-charged engines with more advanced technology have a more favourable cost and CO2 emissions ratio under these terms of comparison. In the light of real driving conditions, the use of a variable inlet valve lift (VVL) or the compres-sion ratio (VCR) appear advantageous. In the Charge-Sustaining-Mode, high tech turbo-charged engines with knock-inhibiting technologies gain clearly, because they drive a heavy vehicle (battery weight) without purely electric driving.

Vehicle Integration Concepts

Package and NVH are of special importance in the integration of the combustion engine into hybrid powertrains. Additional drive components compete for consistently limited space. At the same time, electrification can be employed in an intelligent way with the simplifications and the elimination of existing components. FEV has developed a parametric procedure to be able to evaluate early concepts from a package perspective.

Again, FEV considered the D-segment vehicle with a transversely mounted combustion (“east-west”) engine for the 48V hybrid variants with belt-driven starter generators or starter generators, respectively, as well as with the P2 plug-in hybrid powertrain.

The evaluation of the results revealed that the reference engine (4-cylinder 2.0 liter TC DI) has no significant space disadvantage in the P0 mild hybrid configurations examined. This is primarily due to the elimination of the alternator, which compensates for the additional belt and belt-driven starter generator (BSG). As expected, the ISG variant in transverse mounting is less favorable from a package perspective when comparing the mild hybrids. This is due to the extension in length caused by the ISG and the additional clutch.

For the plug-in hybrid, a critical package parameter value has now been exceeded (“No Go”).

The naturally aspirated 4-cylinder variants reach the critical value already in the 48V mild hybrid variants and exceed it clearly in the plug-in hybrid. The increase in engine length in the transverse mounting is especially critical. The increase of the overall height in particular with regards to the transverse influence on passive pedestrian protection and noise encapsulation measures also reduces the package parameter. This cannot be compensated for by the elimination of the turbocharging components when the transition from the turbocharged to the naturally aspirated engine is made, as these can be placed in a comparatively flexible manner. The package parameter considers this flexibility, which leads to a lower weighting of the turbocharging components compared to other measures – for example, a change of the engine block.

All 3-cylinder variants allow for a significant relaxation of the space problem for the 48V mild hybrid variants. By way of an increase in the degree of downsizing (displacement reduction to 1.3 liter), a small advantage remains even for the plug-in hybrid variant, whereas with a larger displacement (1.5 liter), the 3-cylinder engine is almost neutral compared to the reference powertrain.

Graphics and chart - SI hybrid engines

Gasoline engine technology packages ratio of cost and CO2 emissions for plug-in hybrids (PHEV) compared to the reference powertrain












Noise Vibration Harshness

FEV’s studies show that the NVH requirements for the combustion engine in the hybridized powertrain are controllable if suitable measures are considered in the design concept of hybrid drives. Examples are the increase in starter speed, the integration of balance shafts (or balancing weights with a high balancing degree), the adjustment of the engine mounting and the use of dual mass flywheels. With the help of such measures, 3-cylinder engines can replace 4-cylinder engines in hybrid drives without significant NVH disadvantages.

Emission Calibration Under Real Driving Conditions

The important influence of electrification is the intermittent decoupling of the combustion engine from vehicle propulsion. The connections observed here are more pronounced with an increasing degree of electrification. Therefore, a comparison between the emissions of the powertrains with the highest electrification differentiation was performed.

The highly electrified plug-in hybrid powertrain was examined with two “hybrid-optimized” combustion engines. In view of EU6d limits and RDE, both drives were equipped with a particulate filter. They did not need mixture enrichment for component protection (λ = 1 throughout the map), and had an injection system for reduced particle emissions (turbocharged engine: 350 bar DI and naturally aspirated engine with PFI).
The result: With a full battery, purely electric, emission-free driving led to a clear reduction in route-specific particle emissions for all plug-in hybrid variants in the Charge-Depletion mode. With an empty battery, the particles increased compared to the reference powertrain because the engine had to move the heavier plug-in hybrid vehicle.

Particle Emissions in Electrified Powertrains

Particle emissions in the electrified powertrain (PHEV) shift when compared to the purely combustion engine-driven vehicle dependent on the battery charge status. Generally, the emissions are lowered by emission-free electric driving. When using a plug-in hybrid with an empty battery, a particle emission increase of 18% occurs. The main effect is that purely electric and, therefore, emission-free driving is not possible in that case. In addition, a smaller combustion engine (1.5 liter) is driving a heavier vehicle in the plug-in hybrid compared to the reference vehicle. The higher engine load spectrum increases the particulate raw emissions and the particulate filter slip.

Comparison of NOx Emissions

The emissions for all powertrains also maintain a safe distance from the EU6d limit (even without CF) for NOx and demonstrate behavior analogous to the particle emissions in the comparison of the drives. Therefore, the electrification of the powertrain also lowers the NOx emissions if a fully charged battery and Charge-Depletion mode allow for purely electric driving. Along the same lines, with an empty battery, the NOx emissions increase, as well.

Exhaust Aftertreatment

A significant simplification of exhaust aftertreatment technology is not recommended. An electrically heated catalyst (e-cat) can even be a solution for the trade-off between a maximum electric driving experience with a switched-off combustion engine and regular engine starts for exhaust aftertreatment.

Gasoline engine - SI hybrid engines

Package of a 3-cylinder gasoline engine in the mild hybrid powertrain (48V P2 BSG mild hybrid) with the 4-cylinder reference powertrain


Gasoline engine - SI hybrid engines

Package of a 3-cylinder gasoline engine in the plug-in hybrid powertrain with the 4-cylinder reference powertrain