Focus turns to the Cylinder Head
In a previous Tech Talk, (click here) Elio Motors provided an update on the engine block and cylinder head prototype kick-off. Since then, steady progress continues on the overall Elio Motors engine with the cylinder head being the current focus.
Given fuel efficiency is one of Elio’s four “must haves,” there is a concerted effort applied toward combustion attributes, port design and valve events. With each, there is a strong underlying requirement for reliability, which forces attention to geometric tolerances and interactions, metallurgy, metrology, etc. When the engine is downsized, performance remains important such that the airflow characteristics and capability of the cylinder head/valvetrain are critical.
The Ports
Both the intake and exhaust ports were designed to have unimpeded flow into and out of the combustion chamber. The line-of-sight “tumble port” intake ports are designed to have high-flow capacity as well as generating high levels of in-cylinder air motion. The in-cylinder motion, in concert with careful shaping of the combustion chamber, enables fast burn rates and high knock resistance.
The intake port design and the fuel injector targeting allow for all the fuel to be aimed at the backside of the intake valve via:
Closed Valve Injection Timing: Allows for better fuel vaporization (heating) in part-throttle light load operation
and optimizes combustion efficiency for best fuel economy and lowest emissions.
Value Open Injection Timing: This same fuel targeting allows for very effective strategies at high load full-throttle performance. The fuel enters the chamber in atomized state (liquid – small droplet diameter), which cools the combustion chamber and allows for higher compression ratio and more optimized spark timing. In this way, a more cost efficient port fuel injected (PFI) engine can return similar knock resistance to a direct fuel injection (DI) engine.
The Combustion Chamber
The combustion chamber was designed to be very compact with a low surface/volume ratio. This minimizes the heat transfer out of the combustion chamber, improving the thermal efficiency and consequently improving the fuel efficiency of the engine.
The spark plug is angled towards the center of the combustion chamber and this, together with the deep piston bowl, allows for maximum flame kernel growth prior to surface impingement. This flame growth, in concert with the conversion of bulk in-cylinder motion into flame front turbulence, is responsible for the fast burn rate. A faster burn rate is important for minimizing octane requirements and maximizing higher indicated efficiency.
Another feature of the combustion chamber that enhances the burn rate is the large squish area that was included to help with in-cylinder turbulence near time of ignition.
The features that create the high burn rate also contribute to good homogeneity (uniform mixture composition). High burn rates and good homogeneity allow for high rates of EGR to be applied to the engine, which in turn reduces pumping loss at part-load for improved fuel economy. EGR also reduces the temperatures in the combustion chamber which reduces NOx formation for improved/reduced engine-out emissions.
The Upper Cam Shaft Cover
One of the many innovative design features of the engine is the upper cam shaft cover, which includes integrated cam caps. The integrated cam caps improve cam bearing performance and reduce oil pump requirement/sizing (reduced parasitic loss). This feature also allows for reduced part count in the manufacturing assembly process. The lower part count decreases complexity, which cuts cost. Decreasing the manufacturing complexity minimizes the cycle time for assembly knowing that time is money.
IAV (click here) has engineered the Elio Motors cylinder head to achieve its fuel efficiency goal and has included additional features to minimize overall costs. Stay tuned as future Tech Talks will continue to provide updates on the engine development progress.
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