05-12-2012, 05:21 PM
VTEC TECHNOLOGY-ONE OF THE RECENT ADVANCEMENT IN AUTOMOBILE ENGINE
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ABSTRACT:-
The most important challenge facing car manufacturers today is to offer vehicles that deliver excellent fuel efficiency and superb performance while maintaining cleaner emissions and driving
comfort. This paper deals with i-VTEC(intelligent-Variable valve Timing and lift Electronic Control) engine technology which is one of the advanced technology in the IC engine. i-VTEC is the new trend in
Honda latest large capacity four cylinder petrol engine family. The name is derived from intelligent combustion control technologies that match outstanding fuel economy, cleaner emissions and reduced
weight with high output and greatly improved torque characteristics in all speed range.
The design cleverly combines the highly renowned VTEC system - which varies the timing and amount of lift of the valves -
with Variable Timing Control. VTC is able to advance and retard inlet valve opening by altering the phasing of the inlet camshaft to best match the engine load at any given moment. The two systems work
in concern under the close control of the engine management system delivering improved cylinder charging and combustion efficiency, reduced intak e resistance, and improved exhaust gas recirculation
among the benefits.
i-VTEC technology offers tremendous flexibility since it is able to fully maximize engine potential over its complete range of operation. In short Honda's i-VTEC technology gives us the
best in vehicle performance.
INTRODUCTION:-
An internal combustion is defined as an engine in which the chemical energy of the fuel is released inside the engine and used directly for mechanical work. The internal combustion engine was first conceived
and developed in the late 1800’s. The man who is considered the inventor of the modern IC engine and the founder of the industry is Nikolaus Otto (1832-1891).
Over a century has elapsed since the discovery of IC engines. Excluding a few development of rotary combustion engine the IC engines has still retained its basic anatomy. As our knowledge of
engine processes has increased, these engines have continued to develop on a scientific basis. The present day engines have advances to satisfy the strict environmental constraints and fuel economy
standards in addition to meeting in competitiveness of the world market. With the availability of sophisticated computer and electronic, instrumentation have added new refinement to the engine design.
WORKING OF ENGINE:-
If you have read How Car Engines Work, you know about the valves that let air into the engine and let exhaust out of the engine. You also know about the camshaft that controls the valves. The camshaft uses rotating lobes that push against the valves to open and close them. Check out How Camshafts Work for more information. It turns out that there is significant relationship between the way the lobes are ground on the camshaft and the way the engine performs in different rpm(rotations per
minute) ranges. To understand why this is the case, imagine that we are running an engine extremely slowly -- at just 10 or 20 rpm, so it takes the piston seconds to complete a cycle. It would be impossible to actually run a normal engine this slowly, but imagine that we could. We would want to grind the camshaft so that, just as the piston starts moving downward in the intake stroke, the intake valve would open. The intake valve would close right as the piston bottoms out. Then the exhaust valve would open right as the piston bottoms out at the end of the combustion stroke and would close as the piston completes the exhaust stroke. That would work great for the engine as long as it ran at this very slow speed.
VTEC(VALVE TIMING & ELECTRONIC LIFT CONTROL)
HISTORY:-
VTEC, the original Honda variable valve control system, originated from REV (Revolution-modulated valve control) introduced on the CBR400 in 1983 known as HYPER VTEC. In the regular four-stroke automobile engine, the intake and exhaust valves are actuated by lobes on a camshaft. The shape of the lobes determines the timing, lift and duration of each valve. Timing refers to an angle measurement of when a valve is opened or closed with respect to the piston position (BTDC or ATDC). Lift refers to how much the valve is opened. Duration refers
to how long the valve is kept open. Due to the behavior of the working fluid (air and fuel mixture) before and after combustion, which have physical limitations on their flow, as well as their interaction with the ignition spark, the optimal valve timing, lift and duration settings under low RPM engine operations are very different from those under
high RPM. Optimal low RPM valve timing, lift and duration settings would result in insufficient filling of the cylinder with fuel and air at high RPM, thus greatly limiting engine power output. Conversely, optimal high RPM valve timing, lift and duration settings would result in very rough low RPM operation and difficult idling. The ideal engine would have fully variable valve timing, lift and duration, in which the valves would always open at exactly the right point, lift high enough and stay open just the right amount of time for the engine speed in use.
BASIC V-TEC MECHANISM
The basic mechanism used by the VTEC technology is a simple hydraulically actuated pin. This pin is hydraulically pushed horizontally to link up adjacent rocker arms. A spring mechanism is used to return the pin back to its original position.To start on the basic principle, examine the simple diagram below. It comprises a camshaft with two cam-lobes side-by-side. These lobes drive two side-by-side valve rocker arms.
The two cam/rocker pairs operates independently of each other. One of the two cam-lobes are intentionally drawn to be different. The one on the left has a "wilder" profile, it will open its valve earlier, open it more, and close it later, compared to the one on the right. Under normal operation, each pair of cam-lobe/rocker-arm assembly will work independently of each other. VTEC uses the pin actuation mechanism to link the mild-cam rocker arm to the wild-cam rocker arm. This effectively makes the two rocker arms operate as one. This "composite" rocker arm(s) now clearly follows the wild-cam profile of the left rocker arm. This in essence is the basic working principle of All Honda's VTEC engines.
SOHC(SINGLE OVERHEAD CAMSHAFT) VTEC:-
An alternative implementation of VTEC for high (versus very high) specific output is used in Honda's SOHC engines. SOHC VTEC engines have often been mistakenly taken as a 'poor' second-rate derivative of DOHC VTEC but this is not the true case. An SOHC engine head has advantages of a DOHC head mostly in terms of size (it is narrower) and weight. For more sedate requirements, an SOHC engine is preferable to the DOHC engine. SOHC VTEC is a power implementation of VTEC for SOHC engines with the express intention of extracting high specific output.