Nissan recently published an SAE paper outlining the development of the new engine, which was developed as a new concept for achieving low levels of CO2 emissions. In the engine, the use of a high compression ratio (13:1) with a boosting system, a lower surface/volume ratio piston, and high tumble intake port optimize direct injection combustion. A supercharger with a magnetic clutch compensates for the disadvantage of the small displacement engine with the Miller cycle without sacrificing fuel economy.
The HR12DDR is based on the HR12DE, and betters that engine in terms of power output and torque, as well as fuel consumption and CO2emissions.
| Nissan engine specifications | ||||||
|---|---|---|---|---|---|---|
| MR09ERT | CR14DE | HR12DDR | HR12DE | |||
| Bore (mm) | 66 | 73 | 78 | 78 | ||
| Stroke (mm) | 68 | 82.8 | 83.6 | 83.6 | ||
| Displacement (cc) | 930 | 1386 | 1198 | 1198 | ||
| Comp. ratio | 7.7 | 9.8 | 13 | 10.7 | ||
| Max power (kW/rpm) | 81/6400 | 72/5600 | 72/5200 | 58/6000 | ||
| Max torque (N·m/rpm) | 130/4800 | 137/3200 | 142/4400 | 106/4400 | ||
| Aspiration | super and turbocharger | natural | supercharger | natural | ||
| Injection | port | port | direct | port | ||
| CO2 (g/km) Euro | Jpn only | 154 | 95 | 115 | ||
Partly to address that issue, Nissan adopted the direct injection system and additional cooling items such as a piston oil jet and high thermal conductivity piston ring.
Generally, Nissan noted, a valve recess on the piston surface is required to enhance volumetric efficiency under high load conditions. For the HR12DDE engine, this role can be taken over by supercharging, thereby minimizing valve recess and achieving a minimum piston S/V ratio. As a result, the piston retains its low S/V even with a compression ratio of 13, leading to higher thermal efficiency under partial load conditions. This piston enables Nissan to reach the same level of thermal efficiency at a CR of 13 as a conventional piston with a compression ratio of 14.
To maximize the effect of direct injection (DI), Nissan engineers adopted a 6-hole injector, and determined its spray pattern using computational fluid dynamics (CFD) simulations, considering the oil dilution issue in small bore engines.
While DI offers a number of well-known benefits (knock suppression through using the latent heat of evaporation to lower the temperature and the end of compression, better homogeneous combustion stability), it can also result in a non-homogeneous mixture in the cylinder, resulting in slower combustion, formation of hydrocarbon emissions (HC), and knock. A strong gas flow is required as a countermeasure to improve the homogeneity of the mixture at intake and compression, Nissan said.
Because the new engine has a supercharging system, the intake port could be designed for tumble flow specifically, allocating the air charging function on the supercharger. A sharp edge on the intake port brings on a shredding of the port lower side air flow and increased tumble.
Minimizing pumping loss. The main concept for reducing pumping loss is the late intake valve closing (LIVC) Miller cycle with high compression ratio. This enables a high expansion ratio even in LIVC state. In the HR12DDR, intake valve closing (IVC) is retarded until 100 ° after bottom dead center (ABDC)—corresponding to an effective CR of about 7:1. To reduce the rest of the pumping loss on un-boosted partial load, Nissan combined internal EGR with exhaust continuously variable valve timing (CVTC) and external EGR.
However, this concept causes “a severe combustion state”—inert burned gas and low effective CR. Further, combining Miller and a high CR attenuates the gas flow.
To strengthen gas turbulence at ignition, Nissan adopted a swirl control valve on the intake port; the swirl flow contribues to combustion stability even with heavy EGR.
The Miller cycle with internal EGR is realized using intake and exhaust CVTC; with this engine, Nissan is adopting for the first time an intake retard CVTC system. Nissan says that the system can satisfy both engine stability and LIVC Miller operation for fuel economy.
Reducing mechanical friction. Nissan adopted various friction-reducing technologies on the HR12DDR in addition to the piston oil jet, including H-free Diamond Like Carbon (DLC) coated piston rings—a first.
Although piston ring tension was increased to suppress oil consumption caused by increased thermal load in a high output boosted engine, the piston friction remained the same due to the DLC-coated ring.
A new decouple damper pulley and supercharger electromagnetic clutch and auto tensioner resulted in lower belt tension and reduced the friction by 20% compared to a conventional system.
In spite of the use of supercharging boosting, direct injection, and oil-dependent devices such as exhaust VTC, Nissan achieved a 10% reduction in engine friction with the HR12DDR compared to the HR12DE.
Optimized boosting system. Because the engine is a small displacement Miller engine, it has relatively less exhaust gas volume and energy, Nissan noted, citing that as the reason why the boosting system does not rely on the exhaust gas energy.
Nissan used an Eaton Roots-type supercharger with twin 4-lobe rotors, with a revolution of 2.4:1 compared to engine revolution. Although the supercharger already has low-rotating friction under a non-boosting condition, Nissan adopted the electromagnetic clutch—driven directly by the ECU—for further friction reduction. For boost pressure control, the engine uses a bypass valve upstream of the throttle valve.
An air-cooled intercooler is downstream of the supercharger; Nissan said that this is important to maintain knock quality using Miller cycle on the boost condition. It alo makes it easy to control the charged air volume along with recirculation under partial boost conditions.
Source: Green Car Congress
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