Being the first journalist to test drive the only example of a new piece of technology still in development always concentrates the mind. Doing so in wet and windy weather battering rural roads close to England’s North Sea coast, brings an added dimension to the experience.
But that was how Torotrak demonstrated to Automotive Engineering the potential of its new, CVT-controlled V-Charge pressure charger—a mechanical, gearless, fully variable pressure charger driven by a conventional pulley. It allows compressor speed to vary independent of engine speed. It's designed to provide low-end torque and tackle the effects of turbo-lag on downsized gasoline engines. (See http://articles.sae.org/12401/; also SAE Technical Paper http://papers.sae.org/2015-01-1971/).
Fitted to a 1.0-L Ford Focus as a technology demonstrator (Torotrak is partnering with Ford on this project), it is currently being shown to industry engineers and senior executives across Europe.
As well as general downsizing V-Charge is proving of particular interest to those companies working on Miller-cycle engine applications to meet Euro 7 emissions legislation, says Richard Dunne, Torotrak’s Group Business Development Manager. Independent testing has been conducted by the U.K.’s University of Bath.
Getting something for nothing has always been a problem for engineers. So V-Charge does not reduce fuel consumption of the regular 1.0-L; that is not Plan A, Dunne explained. But compared to the equally regular Ford Sigma 1.5-L it is claimed to lower consumption by up to 12%, with a comparable cut of CO2 emissions.
It does this without recourse to 48-volt mild hybrid technology and uses a standard off-the-shelf (OTS) compressor. Unlike an e-boost system, this provides continuous boosting, asserted Dunne: “It is not complex; the variable ratio traction drive is a scaled down version of our established and proven toroidal CVT technology,.” he noted. System weight is 6 kg (13.2 lb).
The test drive
With the author behind the wheel of the manual-gearbox Focus demonstrator, Dunne used laptop control to switch the system in or out to give direct engine behavior comparisons. The development car uses a turbocharger slightly larger than that of the standard Ecoboost unit to give 110 kW (147 hp), an 18-kW/24-hp) increase.
The efficacy of the V-Charge was apparent on the first steep hill encountered; the initial step-off from an intersection; and a 90° bend tackled with engine speed dropping towards 1000 rpm in 3rd gear. Low engine speed and (relatively) high torque were smooth partners typically without the need for downshifting.
Baseline rated torque for a regular Ford 92-kW (123-hp) triple is 100 Nm (74 lb·ft) from 1000 rpm and 170 N·m (125 lb·ft) from 1400 rpm. With V-Charge the figures rise to 145 N·m (106 lb·ft) and 240 N·m (177 lb·ft) respectively, very similar to those for the 1.5-L 4-cylinder. It is perfectly amenable for C-segment or smaller cars with a curb weight of 1289 kg (2841 lb).
With a CVT ratio spread of 10, an 1100-rpm engine speed can be converted to anywhere between 10,000 and 100,000 rpm at the impeller without any traditional gearing, explained Dunne. The system is designed to provide greatest efficiency from idle to 2500 rpm, tapering off to avoid over-boosting. It provides a ratio change in a maximum 400 ms.
All this happens very quietly; Dunne said the system is 97% quieter than a comparable Roots-type supercharger. It is easily packaged at the rear of the engine. No noise attenuation is required, he claims.
An electric actuator is used to change ratios (there are no control hydraulics), so parasitic losses are minimized. There is no metal-to-metal contact in the CVT which uses traction fluid of proven performance. The ratio spread provides 0.28 to 2.82:1 gearing. When the charger is not needed, the ratio moves to its minimum and there is no need for a disconnect clutch.
Bearing in mind that a non-standard turbo—Dunne would not name the supplier—was fitted to increase maximum engine power, without the V-Charge the test Focus was distinctly lethargic at low revs in higher gears on the test drive.
The system is designed to provide a power capacity of up to 17 kW (23 hp) and can run at that level continuously.
Claimed fuel consumption and emissions for the 1.0-L with V-Charge are 2 - 5% higher than the standard engine due to the added load (about 100 W) caused by the system. However, the figures are estimated to be 12% lower than the 1.5-L's for similar or improved performance.
Cost similar to a VGT
Torotrak is not a manufacturer; it would sell or license V-Charge technology to a Tier 1 supplier. Doug Cross, Torotrak’s Chief Technical Officer, says the V-Charge would represent cost comparable to a variable geometry turbocharger.
“The complete on-cost needs to be considered to determine a cost:-benefit ratio," Cross told AE. "When you sell technology to an OEM, your transfer price is effectively doubled by the time it reaches the customer.” But offsets include cost saved by using a triple instead of a 4-cylinder engine without loss of performance.
It is possible, therefore, that by adding V-Charge only a modest cost penalty might be involved — and even a cost saving, Cross believes.
A former engine design manager for the Renault F1 team and before that with Toyota F1, Cross was co-founder of Flybrid, (now part of Torotrak), which became a specialist KERS (Kinetic Energy Recovery System) developer and supplier.
Ford recently announced that it plans to add cylinder de-activation capability to its 1.0-L triple for MY2018 (see http://articles.sae.org/15130/). Cross believes V-Charge could be used to complement that technology.
One concern regarding use of any CVT was once the behavior of its traction fluid in extremely low temperatures, explains Cross: “We have done a huge amount of development and we now have a traction fluid that is liquid pumpable down to -30º C.”
With many OEMs involved in Miller-cycle engine development (or variations of it), Cross is confident that V-Charge would be compatible with the technology.
Although Torotrak has concentrated on gasoline engine applications for V-Charge, it would also be applicable to turbodiesels, overcoming the negative aspects of transient load steps when the accelerator is floored.
To meet the driver’s demand, either the amount of EGR is reduced, which can bring a NOx spike, or the injection of extra fuel is used to help spin up the turbo which results in unburned fuel in the exhaust.
“So it’s a trade-off between particulates and NOx,” said Cross. “These transient tip-in events are responsible for about 40% of the NOx emissions from a diesel engine. But a V-Charge, running constantly, just changes its ratio to provide added boost. The EGR is unchanged and no fuel is added.”
So far this has not been tested by Torotrak on a diesel engine. But Cross is confident that it would provide an effective solution to the problem of NOx spikes.