I-VTEC Engine - Engineering Seminar

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.

From the past few decades, automobile industry has implemented many advance technologies to improve the efficiency and fuel economy of the vehicle and i-VTEC engine introduced by Honda in its 2002 Acura RSX Type S is one of such recent trend in automobile industry.     

 i-VTEC:-

                The latest and most sophisticated VTEC development is i-VTEC ("intelligent" VTEC), which combines features of all the various previous VTEC systems for even greater power band width and cleaner emissions. With the latest i-VTEC setup, at low rpm the timing of the intake valves is now staggered and their lift is asymmetric, which creates a swirl effect within the combustion chambers. At high rpm, the VTEC transitions as previously into a high-lift, long-duration cam profile.

            The i-VTEC system utilizes Honda's proprietary VTEC system and adds VTC (Variable Timing Control), which allows for dynamic/continuous intake valve timing and overlap control.

The demanding aspects of fuel economy, ample torque, and clean emissions can all be controlled and provided at a higher level with VTEC (intake valve timing and lift control) and VTC (valve overlap control) combined.

The i stands for intelligent: i-VTEC is intelligent-VTEC. Honda introduced many new innovations in i-VTEC, but the most significant one is the addition of a variable valve opening overlap mechanism to the VTEC system. Named VTC for Variable Timing Control, the current (initial) implementation is on the intake camshaft and allows the valve opening overlap between the intake and exhaust valves to be continuously varied during engine operation. This allows for a further refinement to the power delivery characteristics of VTEC, permitting fine-tuning of the mid-band power delivery of the engine.

VTEC ENGINE:

VTEC (standing for Variable valve Timing and lift Electronic Control) does Honda Motor Co., Ltd. develop a system. The principle of the VTEC system is to optimize the amount of air-fuel charge entering, and the amount of exhaust gas leaving, the cylinders over the complete range of engine speed to provide good top-end output together with low and mid-range flexibility.

 VTEC system is a simple and fairly elegant method of endowing the engine with multiple camshaft profiles optimized for low and high RPM operations. Instead of only one cam lobe actuating each valve, there are two - one optimized for low RPM smoothness and one to maximize high RPM power output. Switching between the two cam lobes is controlled by the engine's management computer. As the engine speed is increased, more air/fuel mixture needs to be "inhaled" and "exhaled" by the engine. Thus to sustain high engine speeds, the intake and exhaust valves needs to open nice and wide.As engine RPM increases, a locking pin is pushed by oil pressure to bind the high RPM cam follower for operation. From this point on, the valve opens and closes according to the high-speed profile, which opens the valve further and for a longer time.

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 of Honda's VTEC engines.

DIFFERENT VARIANTS OF V-TEC

VARIABLE TIMING CONTROL (VTC)

                                  VTC operating principle is basically that of the generic variable valve timing implementation (this generic implementation is also used by by Toyota in their VVT-i and BMW in their VANOS/double-VANOS system). The generic variable valve timing implementation makes use of a mechanism attached between the cam sprocket and the camshaft. This mechanism has a helical gear link to the sprocket and can be moved relative the sprocket via hydraulic means. When moved, the helical gearing effectively rotates the gear in relation to the sprocket and thus the camshaft as well.

The drawing above serves to illustrate the basic operating principle of VTC (and generic variable valve timing). A labels the cam sprocket (or cam gear) which the timing belt drives. Normally the camshaft is bolted directly to the sprocket. However in VTC, an intermediate gear is used to connect the sprocket to the camshaft. This gear, labelled B has helical gears on its outside. As shown in the drawing, this gear links to the main sprocket which has matching helical gears on the inside. The camshaft, labelled C attaches to the intermediate gear.

The supplementary diagram on the right shows what happens when we move the intermediate gear along its holder in the cam sprocket. Because of the interlinking helical gears, the intermediate gear will rotate along its axis if moved. Now, since the camshaft is attached to this gear, the camshaft will rotate on its axis too. What we have acheived now is that we have move the relative alignment between the camshaft and the driving cam-sprocket - we have changed the cam timing!

 i-VTEC SYSTEM

Diagram explains the layout of the various components implementing i-VTEC.  I have intentionally edited the original diagram very slightly - the lines identifying the VTC components are rather faint and their orientation confusing. I have overlaid them with red lines. They identify the VTC actuator as well as the oil pressure solenoid valve, both attached to the intake camshaft's sprocket. The VTC cam sensor is required by the ECU to determine the current timing of the intake camshaft.  The VTEC mechanism on the intake cam remains essentially the same as those in the current DOHC VTEC engines except for an implementation of VTEC-E for the 'mild' cam. 

                The diagrams show that VTEC is implemented only on the intake cam.  Now, note that there is an annotation indicating a 'mostly resting (intake) cam' in variations 1 to 3. This is the 'approximately 1-valve' operating principle of VTEC-E. I.e. one intake valve is hardly driven while the other opens in its full glory. This instills a swirl effect on the air-flow which helps in air-fuel mixture and allows the use of the crazy 20+ to 1 air-to-fuel ratio in lean-burn or economy mode during idle running conditions.  On first acquaintance, variations 1 and 3 seem identical. However, in reality they represent two different engine configurations - electronic-wise. Variation 1 is lean burn mode, the state in which the ECU uses >20:1 air-fuel ratio. VTC closes the intake/exhaust valve overlap to a minimal. Note that lean-burn mode or variation 1 is used only for very light throttle operations as identified by the full load Torque curve overlaid on the VTC/RPM graph. During heavy throttle runs, the ECU goes into variation 3 Lean-burn mode is contained within variation-2 as a dotted area probably for the reason that the ECU bounces to-and-fro between the two modes depending on engine rpm, throttle pressure and engine load, just like the 3-stage VTEC D15B and D17A. In variation-2, the ECU pops out of lean-burn mode, goes back to 14.7 or 12 to 1 air-fuel ratios and brings the intake/exhaust overlap right up to maximum. This as Honda explains will induce the EGR effect, which makes use of exhaust gases to reduce emissions.  Variation-3 is the mode where the ECU varies intake/exhaust-opening overlap dynamically based on engine rpm for heavy throttle runs but low engine revs. Note also that variations 1 to 3 are used in what Honda loosely terms the idle rpm. For 3-stage VTEC engines, idle rpms take on a much broader meaning. It is no longer the steady 750rpm or so for an engine at rest. For 3-stage VTEC, idle rpm also means low running rpm during ideal operating conditions, i.e. closed or very narrow throttle positions, flat even roads, steady speed, etc. It is an idle rpm range. The K20A engine implements this as well.  

          Variation-4 is activated whenever rpm rises and throttle pressure increases, indicating a sense of urgency as conveyed by the driver's right foot. This mode sees the wild(er) cams of the intake camshaft being activated, the engine goes into 16-valve mode now and VTC dynamically varies the intake camshaft to provide optimum intake/exhaust valve overlap for power.                                         

           On i-VTEC engines, the engine computer also monitors cam position, intake manifold pressure, and engine rpm, then commands the VTC (variable timing control) actuator to advance or retard the cam. At idle, the intake cam is almost fully retarded to deliver a stable idle and reduce oxides of nitrogen (NOX) emissions. The intake cam is progressively advanced as rpm builds, so the intake valves open sooner and valve overlap increases. This reduces pumping losses, increasing fuel economy while further reducing exhaust emissions due to the creation of an internal exhaust gas recirculation (EGR) effect.

              i-VTEC introduced continuously variable timing, which allowed it to have more than two profiles for timing and lift, which was the limitation of previous systems. The valve lift is still a 2-stage setup as before, but the camshaft is now rotated via hydraulic control to advance or retard valve timing. The effect is further optimization of torque output, especially at low RPMs. 

 Increased performance is one advantage of the i-VTEC system. The torque curve is "flatter" and does not exhibit any dips in torque that previous VTEC engines had without variable camshaft timing. Horsepower output is up, but so is fuel economy. Optimizing combustion with high swirl induction makes these engines even more efficient.                                                          Finally, one unnoticed but major advantage of i-VTEC is the reduction in engine emissions. High swirl intake and better combustion allows more precise air-fuel ratio control. This results in substantially reduced emissions, particularly NOx. Variable control of camshaft timing has allowed Honda to eliminate the EGR system. Exhaust gases are now retained in the cylinder when necessary by changing camshaft timing. This also reduces emissions without hindering performance.

APPLICATIONS                                                                                                                                   

Currently i-VTEC technology is available on three Honda products;

  2002 Honda CRV

  2002 Acura RSX

  Honda Civic 2006

CASE STUDY  OF ‘HONDA  CIVIC  2006’  WITH  1.8 liter  ENGINE

          The new i- VTEC system in Honda civic 2006 uses its valve timing control system to deliver acceleration performance equivalent to a 2.0-liter engine and fuel economy approximately 6% better than the current 1.7-liter Civic engine. During cruising, the new engine achieves fuel economy equivalent to that of a 1.5-liter engine.

          In a conventional engine, the throttle valve is normally partly closed under low-load conditions to control the intake volume of the fuel-air mixture. During this time, pumping losses are incurred due to intake resistance, and this is one factor that leads to reduced engine efficiency.

             The i-VTEC engine delays intake valve closure timing to control the intake volume of the air-fuel mixture, allowing the throttle valve to remain wide open even under low-load conditions for a major reduction in pumping losses of up to 16%. Combined with friction-reducing measures, this results in an increase in fuel efficiency for the engine itself.

            A DBW (Drive By Wire) system provides high-precision control over the throttle valve while the valve timing is being changed over, delivering smooth driving performance that leaves the driver unaware of any torque fluctuations.

          Other innovations in the new VTEC include a variable-length intake manifold to further improve intake efficiency and piston oil jets that cool the pistons to suppress engine knock.

         In addition, lower block construction resulting in a more rigid engine frame, aluminum rocker arms, high-strength cracked connecting rods, a narrow, silent cam chain, and other innovations make the engine more compact and lightweight. It is both lighter and shorter overall than the current Civic 1.7-liter engine, and quieter as well.

 SPECIFICATIONS  OF   1.8l i-VTEC ENGINE

  Engine type and number of cylinders         Water-cooled in-line 4-cylinder

  Displacement                                               1,799 cc

  Max power / rpm                                          103 kW (138 hp)/ 6300

  Torque / rpm                                                 174 Nm (128 lb-ft)/4300

  Compression ratio                                        10.5:1

 PERFORMANCE 

This new engine utilizes Honda's "VTEC" technology, which adjusts valve timing and lift based on the engine's RPM, but adds "VTC" - Variable Timing Control - which continuously modulates the intake valve overlap depending on engine load. The two combined yield in a highly intelligent valve timing and lift mechanism.In addition to such technology, improvements in the intake manifold, rearward exhaust system, lean-burn-optimized catalytic converter help to create an engine that outputs 103kW (140PS) @ 6300rpm,and provides ample mid-range torque. It also satisfies the year 2010 fuel efficiency standard of14.2km/Landreceives the government standard of "LEV" .

 FUTURE TRENDS                                                                                                    

               From now onwards, there is all likelihood that Honda will implement i-VTEC on its performance engines.  Again what i-VTEC does allow is for Honda to go for the sky in terms of specific power output but yet still maintaining a good level of mid-range power. Already extremely authoritative reviewers like BEST motoring have complained about the lack of a broad mid-range power from for e.g. the F20C engine. In a tight windy circuit like Tsukuba and Ebisu, the S2000 finds it extremely tough going to overtake the Integra Type-R in 5-lap battles despite having 50ps or 25% more power. To get the extreme power levels of the F20C, the wild cams' power curve are so narrow that there is effectively a big hole in the composite power curve below 6000rpm. What i-VTEC can do to this situation is to allow fine-tuning of the power curve, to broaden it, by varying valve opening overlap. Thus this will restore a lot of mid-range power to super-high-output DOHC VTEC engines allowing Honda, if they so desire, to go for even higher specific outputs without too much of a sacrifice to mid-range power.           

CONCLUSION      

 i-VTEC system is more sophisticated than earlier variable-valve-timing systems, which could only change the time both valves are open during the intake/exhaust overlap period on the transition between the exhaust and induction strokes. By contrast, the i-VTEC setup can alter both camshaft duration and valve lift.  i-VTEC Technology gives us the best in vehicle performance.  Fuel economy is increased, emissions are reduced, derivability is enhanced and power is improved.