|Speedstart liquid-cooled switched-reluctance motor-generator. Click to enlarge.|
UK-based Controlled Power Technologies (CPT) has completed more than two years of continuous testing to validate its SpeedStart belt-integrated starter-generator for 1.2 million stop-starts—considered the standard that will be required for a new generation of micro-mild hybrid vehicles. Developed from the outset for 12, 24 and 48 volt applications, CPT’s SpeedStart system is the first liquid-cooled switched-reluctance motor-generator developed for automotive stop-start. (Earlier post.)
Conventional starter motors typically look at 30,000 stop-starts, while current generation stop-start systems target up to 300,000 events. CPT said that no issues surfaced with the technology in any of the recorded test data throughout this extended period of stop-start testing, which was followed by a teardown and forensic examination of CPT’s compact motor-generator system.
A stop-start system has to be specified for near-future requirements including extending vehicle life and increasingly demanding hybridization strategies, and a typical modern car moreover can easily last 250,000 miles over 15 years. More critically, fuel economy targets require reduced stop-inhibits, not just stopping the engine when the driver places the transmission in neutral. Latest implementations allow frequent stop-start events in crawling traffic and future coast-down strategies. Switching off the engine, but preparing for immediate re-starts if the driver demands acceleration, will further increase the frequency of stop-starts.
We needed therefore to demonstrate as convincingly as possible the near-zero probability of failure during the lifespan of a vehicle as well as the ability to achieve the maximum number of re-starts, because one of the most cost effective solutions for low fuel consumption is simply stopping the engine at every single opportunity—even if it’s only for a few seconds, every stop counts.—CPT hybrid product group manager Peter Scanes
The sealed unit with integrated control and power electronics avoids any ingress of dirt; the liquid cooling is plumbed into an engine’s coolant system for thermal management, said Al Muncey, senior engineer for SpeedStart mechanical and electrical systems at CPT and lead engineer on the project.
The first engine stop-start test began on 15 October 2010 and continued around the clock for 24 hours a day seven days a week until 1.2 million restarts had been achieved almost a year later on 30 September 2011. The test schedule demanded a restart on average every 12 seconds, with the actual time between starts varying between 5 and 25 seconds—equivalent to 300 an hour or 7,200 re-starts every single day for a total duration of 350 days of continuous testing.
Before this initial validation programme had been completed, another test was commissioned on 2 June 2011 with a second SpeedStart unit, which from September 2011 onwards was tested 24 hours a day for five days a week until another 1.2 million restarts had been achieved almost two years later on 15 February 2013. Testing of this second unit will now continue until it does ultimately fail to establish just how many restarts can be achieved and the likely maximum service life.
In addition to its start-stop capability, the SpeedStart technology has been designed from the outset to provide significant brake energy recuperation and is an efficient motor-generator. Despite the frequency at which the engine was continuously stopped followed by immediate re-starts, and with a regularity which the average motorist is unlikely to experience even in the most heavily congested urban traffic, the SpeedStart units still had sufficient time to generate more than three times the amount of electrical energy required to restart the engine following each stop event.
Over the two-and-a-half year test period the two SpeedStart units regenerated 35 Gigajoules of electrical energy—approximately 10MWh. In terms of chemical energy it’s the equivalent of combusting six barrels of oil.
For a vehicle to recover that much energy it requires an efficient electrical machine, which is almost as important as exceeding the industry’s durability requirements. And when you do eventually reach the end of life of the vehicle, it’s equally important that all the material can be easily recovered and recycled. And this is where switched reluctance machines have another major advantage, because they eliminate the need for permanent magnets made from expensive rare earth materials—leaving only steel, copper and aluminium and small amounts of plastic and silicon in the electronic components to be recovered. Consequently, the recyclability of our machines is virtually 100%.—Nick Pascoe, chief executive
Multiple SpeedStart units are also undergoing accelerated testing to represent a lifetime of generation in the hostile under-hood environment, cycling between -25 and +125⁰C air temperature, between -25 and +110⁰C engine coolant temperature and between zero and full electrical loads.