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Technical Overview of Honda's new R18 i-VTEC Implementation

As explained above, under so-called low-load conditions (steady speed cruising within speed limits, down-hill, generally light to very light throttle driving) on a conventional engine, the throttle valve is normally partly closed to control or limit the intake volume of the fuel-air mixture. The air-flow restriction that is incurred due to intake resistance in such conditions is one of the biggest factor that leads to reduced engine efficiency, through pumping losses explained above. With the R18A SOHC i-VTEC mechanism, Honda's R&D engineers introduced the idea of controlling the amount of air entering the combustion chamber via other means instead of the throttle butterfly. This is done via a combination of both valve timing and an exploitation of the property of the 4-stroke cycle and a new Drive By Wire (DBW) system.

First, the intake valve closure timing is delayed to a very long period, way into the piston upstroke part of the cycle. The idea is to allow air to enter the cylinders freely, without using the throttle butterfly to artificially restrict air-flow. Then by opening the intake valves right through to the piston upstroke part of the cycle, the excess amount of air-fuel mixture that is in the cylinder will be expelled by the upward stroke of the pistons out through the intake valves and back into the intake manifold. When the amount of air-fuel mixture left remaining in the cylinder/combustion chamber is the desired amount, only then will the intake valves be closed. So instead of using the throttle butterly to control or limit the amount of air entering the engine, we allow the air to enter the cylinders and then push the unwanted amount out of the cylinders. So the exact amount of air we want is now controlled by the timing of the intake valve closure. In this way, the throttle valve/butterfly is not used/needed and it is allowed to remain wide open even under low-load conditions. This contributes to a a major reduction in pumping losses - up to 16% reduction according to Honda's calcuations. Combined with comprehensive friction-reducing measures, this results in a significant increase in fuel efficiency for the engine itself.



The new DBW (Drive By Wire) system on the R18A now constitutes a major and crucial part of the R18A's SOHC i-VTEC mechanism. Firstly, it provides the high-precision control over the throttle valve required while the valve timing is being changed over, ensuring smooth driving performance that leaves the driver unaware of any torque fluctuations. Secondly, and more importantly, during the economy mode there is the unusual, and conflicting situation where while the throttle pedal is at an almost completely closed position, the throttle butterfly in the throttle body is in an almost completely opened position, the exact opposite ! However, this relationship only applies when the engine is running in economy mode, when VTEC has activated the fuel economy cam lobes. In normal running, the normal relationship between throttle pedal position and throttle butterfly opening remains, i.e. light throttle means small throttle butterfly opening, etc. Thus the SOHC i-VTEC mechanism used in the R18A is only possible with a DBW system.

So the new SOHC i-VTEC implementation is an entirely new implementation, quite unlike those already in use. Under normal driving conditions, the R18A runs on its normal set of cam-lobes which in a completely reversed role, Honda calls the 'hot-cams'. So VTEC-off on the R18A means it can be considered to be running high cams. When the right conditions are acheived for fuel economy, VTEC engages the 2nd set, the 'low' or 'economy' cams. Thus VTEC-on on the R18A means it is running low cams. More importantly is VTEC engages the low-cams only if the right conditions for fuel economy are acheived. If they are never acheived, like when we are driving aggressively for the whole trip for e.g., VTEC will never open on the R18A. Thus it is a completely different idea from the VTEC implementations of old where VTEC will always open after a certain rpm, irregardless of the driving conditions. This is also the reason why Honda feels it is justified to call this new implementation an i-VTEC implementation, 'i' of course meaning 'intelligent'. This is because the engine/ECU is 'smart' enough to know if conditions are right or not for VTEC to engage. This new mode of operating is clearly illustrated in the 3-dimensional torque-rpm-driving load chart on the left (supplied by Honda). Note the standard torque curve. This is the line right at the top of the chart and VTEC never activates across this torque curve. On the other hand, VTEC opens across a fairly wide range of driving conditions, in the shaded portion inside the torque curve. So if our throttle pedal position or other critical driving conditions are outside the shaded portion, the R18A will not engage VTEC at all.



Now normal driving conditions means a torquey and quite powerful R18A engine, thanks to the relatively aggressive high (low) cams and the new variable length intake manifold and other friction reducing technologies in the engine. So the 1.8l R18A which is SOHC in configuration is able to deliver as much power as a typical 'performance' oriented 1.8l DOHC engine without any worries about compromising fuel economy. On the other hand, once the driving conditions permits, the ECU 'opens' VTEC on the R18A. Now the reduction in pumping losses acheived allows the R18A to sip fuel at a similar rate to a normal 1.5l engine.

According to Honda, they have managed to design the R18A to deliver an off-the-line acceleration performance equivalent to a 2.0-liter engine while also delivering a fuel economy approximately 6% better than the current 1.7-liter Civic engine (through its low friction loss technologies). This makes it one of the world’s most efficient 1.8-liter engine designs. In addition, when driving conditions are right, like during cruising, the new engine achieves particularly high fuel economy, on a par with that of a 1.5-liter engine when VTEC activates the fuel-economy mode.