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Are Stop-Start Engines Wearing Thin?


Are Stop-Start Engines Wearing Thin?

Since the United States Congress passed the Clean Air Act in 1970 requiring a 90 percent reduction in emissions from new cars by 1975, the auto industry has poured trunk loads of time, energy and resources into creating ever more efficient engines. The 1975 deadline also brought the Energy Policy Conservation Act, which set the first fuel economy goals, and the Corporate Average Fuel Economy program, which laid out a timeline for phasing in even more stringent standards. This and continued legislation around the globe has driven nearly constant innovations in engine design in homage to the all-important miles-per-gallon ratings.

One of these design changes is stop-start technology. In concept, it is a relatively straightforward idea: When the vehicle comes to a complete stop, as at a red light or in traffic, the engine shuts off. When the brake is released and the accelerator is depressed, it starts up again.

Most consumers are familiar with this fuel-saving strategy in hybrid electric vehicles, where it is ubiquitous. Volkswagen first commercialized the technology for passenger cars in the early 1980s with its Formel E engines in the Passat, Polo and Golf models. General Motors said in 2016 that the feature would be available in almost every one of its light vehicles by this year. Most major automakers around the world have incorporated some form of the technology into their passenger car fleet, including but not limited to Bentley, BMW, Citroen, Jaguar Land Rover, Opel, Renault and Volvo in Europe; Fiat Chrysler and Ford in North America; and Hyundai, Honda, Kia, Mazda and Toyota in Asia.

The popular misconception for internal combustion engines is that start-up burns more fuel than idling. But in an engine that has reached optimum operating temperature, restarting doesnt use a large quantity of fuel and can, in fact, save fuel-even without a hybrid system.

Under average driving conditions, stop-start technology can produce 3 to 5 percent fuel savings, Robert Fascetti told the New York Times in its Wheels column in 2016. But with a lot of stops and traffic lights that stay red for extended periods, the added fuel economy can rise to 10 percent, said Fascetti, who was then vice president for powertrains at Ford.

At some point, virtually every vehicle will have stop-start, Ulrich Muehleisen, former head of marketing and product development for stop-start technology provider Robert Bosch, proclaimed in the Times column. However, the experience of the engine cutting in and out seems to grate on the nerves of many drivers, so newer models are hitting the market with a button to disable the feature.

Another concern weighing on the minds of consumers and industry stakeholders is whether or not frequent stop-start cycles cause additional wear in the engine.

We get a lot of work through on stop-start engines, said Peter Lee, principal engineer-tribology at Southwest Research Institute, during the Philadelphia chapter of the Society of Tribologists and Lubrication Engineers George Arbocus Education Course in May. He even went so far as to recommend that attendees turn off the feature in their cars, if possible.

The Coordinating Research Council-a nonprofit supported by the petroleum and automobile industries and run by committees of technical experts from industry and government-developed a test program last year that examined this concern as part of a larger study. The results were published in January as CRC Report No. AVFL-28, Gasoline Direct Injection (GDI) Engine Wear Test Development. A team at Southwest Research Institute, led by Lee, conducted the testing.

The test protocol was set up in an effort to help guide new engine lubrication wear test development in the next passenger car engine oil specifications from the International Lubricants Standardization and Advisory Committee in North America as well as the European Automobile Manufacturers Associations ACEA standards. Until now, wear tests for engine oils were based on operating conditions typical of port fuel injection engines, the independent laboratory explained in its report. Industry trends are moving away from PFI engine design in favor of gasoline direct injection, and stop-start technology can subject the engine and lubricants to even higher stress.

SwRI designed a method to evaluate wear in the rings, liner and rod bearings of a modern, turbocharged GDI engine. The tests were performed with the same Ford EcoBoost engine that is used in the Sequence IX test for low-speed pre-ignition, which is now part of the API SN Plus engine oil service category. These parts were irradiated using radioactive tracer technology, and researchers measured the level of radioactive particles in the oil due to wear of the irradiated engine components. Different isotopes were formed according to the material from which each component was made, allowing particles from each part to be differentiated.

Lee and his team developed a test matrix, running the engine through a series of simulated in-field operating conditions that could be expected to create wear of the chosen components. Their goal was to determine the engine conditions that created wear in different parts of the engine, which could guide future wear test development.

The team chose test conditions that represented what they expected would be the most severe wear conditions in the field. Among them, three sets of stop-start conditions were used: very cold, hot and four-hour hot temperatures. The test cycles began with a hot start, then an immediate hard acceleration at high load to moderate speed for 20 seconds, then a drop to low load for 10 seconds at the same speed. The engine was then stopped and given a one-minute hot soak, and the cycle was repeated. Hot engine temperatures were set at 95 degrees Celsius for the oil gallery, 90 C for the coolant and 35 C for the charge air. The very cold cycle set oil gallery temperature at 25 C, coolant at 35 C and the charge air at 15 C.

All operating conditions were tested with both an SAE 5W-30 oil, which is the factory-recommended viscosity for the EcoBoost engine, and an SAE 0W-16 oil-a viscosity that is beginning to make inroads in the North American marketplace. The oils were composed of the same base oil and additive package. The engine was flushed with the next oil to be used after each test was completed, alternating between the two viscosity grades for each set of operating conditions. Testing the oils in the same conditions back-to-back allowed for a direct comparison of the viscosity grades at the same stage of engine wear.

The results clearly pointed to certain operating conditions that resulted in substantial wear on the irradiated engine parts. Transient operating conditions created more wear than steady state conditions, and the SAE 0W-16 oil produced higher wear than the SAE 5W-30 oil in about two-thirds of the operating conditions. The CRC report also stated that, The stop-start cycles produced some of the most significant differences in wear rates for the two lubricants and often the highest wear rates recorded.

The stop-start cycles produced measurable wear in all but the connecting rod bearing, which did not experience wear in any of the test conditions. It should be noted, however, that the four-hour hot temperature stop-start cycle only produced wear on the second ring face, while the hot stop-start and very cold stop-start conditions produced wear on all of the irradiated components.

For hot stop-start conditions, results showed no wear on the second ring face or top ring side with the SAE 5W-30, but wore about 0.17 micrograms per hour on the second ring face and 288 micrograms on the top ring side. Top ring face wear was more than eight times higher with the SAE 0W-16 oil and nearly four times higher on the cylinder liner, under the same operating conditions.

In the very cold stop-start cycle, the lower-viscosity oil produced 120 percent more wear on the second ring face, 80 percent more on the cylinder liner, 58 percent more on the top ring side and about 43 percent more wear on the top ring face.

These numbers could be cause for alarm, but both the oil industry and OEMs are satisfied that the results are what would be expected.

The report is an interesting read with no real surprises, said Bob Sutherland, Shells passenger car motor oil global technology advisor. He noted that Ford recommends an SAE 5W-30 oil for the EcoBoost engine for that model year, so lower wear with the SAE 5W-30 is not unexpected. He also pointed out that the paper doesnt discuss whether either of the wear rates would create problems for the engine over time.

If I were to ask CRC one question, I would ask if theengine they tested contained any mitigation strategies for stop-start wear concerns such as IROC bearings, oil pressure check valves, etc., commented another industry insider. As for API, stop-start concerns have never surfaced to the point of a request by OEMs that a test be developed.

Angela Willis, who is materials technology engineering group manager at General Motors and chair of the ASTM Passenger Car Engine Oil Classification Panel, said it would be incorrect to assume that increased wear moving from an SAE 5W-30 to an SAE 0W-16 oil is caused by the stop-start conditions. Far more importantly, CRCs test results demonstrate how important it is to use the manufacturers recommended engine oil viscosity.

Engine design is tightly tied to viscosity grade, Willis continued. That means building engines for which SAE 0W-16 and thinner oils are recommended with components like pumps and bearings that are designed to ensure proper oil flow for low-viscosity oils. This is the reason, she noted, that ILSAC has considered creating a symbol distinct from its iconic Starburst for SAE 0W-16 oils.

While OEMs dont see stop-start technology as a threat to their engines, the CRC study seems to indicate that viscosity plays a more important role in protecting such engines from wear.

Industry consultant Steve Swedberg has over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society, ASTM International and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at steveswedberg

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