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Technological innovations in heat pump systems 279 Figure 21. SPF improvements during the last 25 years in heat pumps and air conditioners. at 3 or 4 cylinders, water cooled with thermal recovery on the exhaust and the cooling water, mechanical power of about 7–8 kW. The great novelty is due to the maintenance intervals that arrive at 10,000 hours. The same interval is claimed for the lubricating oil change. This extension is surprising: it would be like a car chang- ing the oil every 300,000 or more km! Some improvements were made to the motor to increase its efficiency. Some manufacturers applied the Miller cycle that already some Japanese car producers use. The Miller cycle is used in supercharged motors and it is similar to the Otto cycle, except for the intake phase. This terminates at the Bottom Dead Center (BDC) in Otto cycle. Intake valve duration is from two degrees before Top Dead Center (TDC) until 70 degrees after BDC in Miller cycle, that is the intake valves remain open for around an additional 30o of crankshaft rotation beyond ‘normal’. This delays the true compression phase of the mixture and a fraction of the intake charge is blown back out the intake valves. The charge loss and the volumetric efficiency reduction is more than compensated by the supercharging, usually obtained by a volumetric compressor (not a turbocharger): a screw compressor is usually used. Compression phase takes place after intake valve shut. The effective compression ratio is reduced from 10 : 1 to about 8 : 1. Also piston compression work is reduced. Motor specific power loss is low, while its operation is smoother with a lower pro- duction of NOx. Combustion process is better, so much the more because the charge is hotter than an intercooled supercharge. The main point is that for the higher compression efficiency of a screw compressor compared to the piston compression the compression work is reduced by 10 to 15% with an improvement in overall motor efficiency of a few percent [9]. Improvements have also been made to the motor control system. The motor can operate at a variable speed so that heat pump capacity is modulated according to the demand. Defrosting is no longer necessary as outside air batteries are set just over International Journal of Low Carbon Technologies 2/3 Downloaded from https://academic.oup.com/ijlct/article/2/3/262/758006 by guest on 21 June 2022PDF Image | Technological innovations in heat pump systems
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