Low-friction techniques push advanced city-car project toward 84-mpg target

  • 02-Dec-2015 01:45 EST
Shell11-15 ProjectMGeoTechpiston.jpg

The piston is the particular focus for attention to improve engine efficiency, explained Geo Technology's Hidehito Ikebe.

In the beginning there were Shell, Gordon Murray Design (GMD), Geo Technology, and an idea.

The idea was to create a very small gasoline engine that demonstrated super efficiency to provide a 3-seat, aerodynamic, lightweight and ultra-compact city car having forward opening single canopy access with zero-width swing. The car would have a combined fuel consumption of 2.8 L/100 km (84 mpg equivalent) or less, and a top speed of around 130 km/h (80 mph).

To achieve it, in April 2015 Project M (for mobility) was initiated and the partners agreed: “Let progress commence.”

It did, with R&D development of the Gordon Murray T25M. Now, as the project continues, the cloak of confidentiality has been partially pulled aside. At Shell’s Hamburg [Germany] Technical Center, senior executives of the companies involved revealed the shape of graphs to come.

Bob Mainwaring, Shell Lubricants Technology Manager for Innovation, said: “Shell supplied lubricants for Gordon Murray Design’s T25 city car program (http://articles.sae.org/10006/) in 2010, with the result that fuel consumption improved by 6.5% in the European urban cycle and 4.7% in the combined cycle. Those results seeded the thinking that with energy consumption set to double by 2050, what would be the result if we worked together on an holistic, co-engineered approach with a new engine design to achieve a very high level of engine efficiency?”

The answer is emerging, with a major advance centering on reducing the friction of almost all moving parts of that engine using advanced lubrication solutions complemented by extensive use of Diamond-like Carbon Coatings (DLC).

Mainwaring emphasized the need for companies to collaborate and to link hardware design with programs to reduce friction. That is well enough known but achieving it can be tough, he acknowledged.

“On one hand we want to reduce viscosity to lower drag forces and lower friction but on the other hand reducing viscosity, oil thicknesses within a component are likely to drop, making the hardware more vulnerable to wear," he noted. "Balancing the two is very important and we call that being ‘close to the edge but safe’.”

"Safe" in this context means pushing the limits of viscosity while achieving acceptable wear. Shell engineers adjusted the lubricant's formulation to give as low viscosity as possible at low temperature, while keeping it as high as it needs to be (which might be a low value at high temperature) to stop surfaces touching and wearing, he said, noting critical interfaces such as the cam-to-tappet and piston ring-to-cylinder liner.

By coating these surfaces with DLC (Diamond-like Compound), the Shell team was able to protect these and ensure that the lubricant-vs.-wear pinch point is the crankshaft and its journal bearings.

That pinch-point is in the control of the viscosity of the lubricant and the additives, Mainwaring explained, adding a simple summation: How low can the viscosity be for the loads that are imposed on the crankshaft?

An essential element of achieving this balance is to consider the lubricant as a design component: “Changing the hardware to match the lubricant and the lubricant to match the hardware will give the best fuel economy,” he told Automotive Engineering.

The prototype formulation for Project M is in the SAE 0W-12 range compared to the typical 5W-30 grade. Industry specifications do not recognize either 0W-12 or thicker 0W-16. In working with Geo Technology to co-engineer the engine and its oil, Shell engineers are essentially throwing away the specification book and the compromises it forces. Their aim instead is to design an engine oil that delivers the very low friction results that it is targeting. Use of friction modifiers, some containing elements such as molybdenum will be used.

It is this bespoke approach that has been applied to the base 660-cm3 Mitsubishi 3-cylinder engine that will power the one-off technology demonstrator T25M. The base engine is high revving and powerful (55 kW/73-hp at 7500 rpm) for its size, but Hidehito Ikebe, Director of Engineering at Geo Technology, said that to support the project’s criteria, this has been reduced to 36 kW (48 hp) at 5500 rpm. Compression ratio was raised from 10.8:1 to 12.4:1.

The company has designed some new parts, introduced "beehive" type valve springs, titanium valves, and incorporated DLC solutions to reduce friction in several areas.

Ikebe said the piston is the most significant area for reducing friction. Except for the crown, piston design for the T25M is all new. Two rings instead of the normal three are used, and the number of strakes reduced by 40%. The connecting rods were extended by 9%, and piston weight reduced by 40%. Collectively these changes produce a 30% reduction in inertial mass, he explained.

There is no intention at present of bringing the T25M to market. Project M is a capability analysis exercise only, but all those involved in it clearly hope that it will represent a building block for future advances in efficiency for series produced engines. Certainly some “insights of value” have emerged, agree the participants.

The project is being funded by Shell, although no specific details of its budget have been released.

The timing of getting the T25M to its unveiling stage (moved from an officially announced November 2015 to Q2 of 2016) is due both to the project being “very ambitious” and for the decision to first gather all the data needed and compare it against three vehicle archetypes on an energy and fuel consumption basis.

Aerodynamics and materials also are a significant part of Project M. With regard to aerodynamic efficiency, it might be thought that it is not of paramount importance for a city car. But Andy Jones, GMD’s Design Director, emphasized that for a car carrying three, 95th-percentile adults, its sub-0,30 Cd figure is good. The vehicle is relatively high at 1.6 m/5.2 ft, but very short for “nose-in” parking at 2.65-m long and 1.35-m wide (8.7 x 4.4 ft) to permit three of the type to park in a standard parking space.

Achieving the relatively low Cd is significant, Jones explained, because city cars are often driven not just in central areas but into and around urban areas where speeds reached can be relatively high. The car's designated 128 km/h (80 mph) Vmax capability makes aerodynamics significant.

Jones said that application of advanced materials and rethink of the vehicle’s primary structure is the second major element of the T25M’s “fight to reduce vehicle drive energy,” with a lightweighting philosophy "that goes beyond conventional architectures.”

The T25M contains the materials evolution that has led to GMD’s iStream Carbon, shown for the first time at the 2015 Tokyo Motor Show. It is the chassis technology used for the Yamaha Sports Ride Concept.

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