Focus on Torque Fill

The FOCUS of this article is on TORQUE FILL systems. It explains the need for these systems, what they do and how the various solutions achieve the desired result.

Two driving modes occur during normal driving: acceleration and constant speed. The amount of power (and thus torque) needed for acceleration is considerably larger than for driving at a constant speed.

Internal combustion engines (ICEs) are almost without exception optimised for minimal fuel consumption during prescribed driving cycles such as the NEDC, which feature mostly moderate acceleration and constant speeds. ICEs, especially the spark ignited (SI) ones, are tuned for high specific outputs to minimise pump losses. The useful range of engine speeds is limited, meaning that the engines are not very flexible (= useful torque spread). More torque is usually needed in real life than available, resulting in using higher engine speeds to achieve the desired acceleration. This is also the main reason why so many vehicles can not meet their fuel consumption figures.

There are various strategies to complement the ICE torque, which is often called Torque fill.

Many current SI ICEs are not naturally aspirated, have forced induction. The boost pressure of these systems is related to the torque output. To achieve high boost pressures on turbocharged engines at low engine speeds, many strategies have been tried.

Some companies (e.g. SAAB) made the turbocharger always generate too much boost and used a waste-gate valve to control the amount of boost. Water injection was used in F1 racing to increase the mass flow to the turbocharger turbines, resulting in higher boost pressures. The higher boost pressure was usually still achieved mostly at higher engine speeds and the slow throttle response (turbolag thanks to the thermal rather than mechanical connection) made it hard to use. An additional high pressure storage tank powered by an ICE has even been invented, but not yet implemented.

To make turbochargers respond quicker and to work at lower exhaust gas flows, strategies as variable exhaust gas flow manipulation via variable vane position or twin-scroll housings are used. More often a compound system of multiple turbochargers is used, in which the turbochargers can be configured parallel, in series or a combination. Some brands have already used three turbochargers on one ICE, such as BMW on its N57S I6 diesel.

A combination of a mechanically driven supercharger and turbocharger, such as the VW TwinCharger system or the high performance Zenvo ST, seems to offer the best of both worlds. Better still though would be a compressor which would be driven independent from the ICE. The Ricardo HyBoost system uses an electrically driven compressor powered via supercapacitors, which could combine well with the Mazda i-ELOOP system. Audi plans to use a 48V battery for this purpose.

On naturally aspirated (NA) engines the common methods for increasing torque is to use variable valve lift and timing, as well as variable inlet and exhaust system pressure manipulation. The only other option which is used to complement a NA engine is to combine it with an electric motor into a parallel or series hybrid powertrain.

The torque fill is what make the new generation of hypercars hyper (responsive). In the Porsche 918 Spyder e-motors are used to power both the front and rear axle, of which the one on the rear axle also functions as the main generator. In the McLaren P1 only the rear axle is driven by a TC ICE with the e-motor/generator integrated in the ICE. In the LaFerrari the HY-KERS system uses an e-motor connected to the gearbox rather than integrate in the ICE.

As can be seen in the new generation of hypercars, increased torque can be realised without harming the throttle response. With the rise of Plug-in Hybrid EVs featuring the required hardware for torque fill and the simplicity of the HyBoost system, it seems clear that torque fill will become standard and extend the life of the ICE a little more. ¤

This article first appeared on my now defunct website www.autoweekly.info on 2014-07-15.

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