Automotive Engineering - April 2021 - 22

Optimizing hybrids

FOR COST AND
EFFICIENCY
Figure 5: Modular engine
friction-reduction measures.

Cam Carrier
Valve Train Module

Variabilities - DVVT / VVL

Cylinder Head Module

Miller and Performance Variants
Atkinson NA Variant
VCS System
Compression ratio spread up to 6 units / up to 6500 rpm.

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Fastest and reproducible switching behavior

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No additional energy demand for VCS control in fuel economy position (high CR).

Figure 6: Components of AVL's variable-compression system.
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.
22 April 2021

Figure 7: Modular cylinder head and valve train assembly.
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
AUTOMOTIVE ENGINEERING

ALL IMAGES: AVL

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https://www.sae.org/publications/technical-papers/content/2017-01-0634/

Automotive Engineering - April 2021

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