Xjyuk

New cars have StopStartSystems ( even though was available for long time. My question is, why car companies are emphasizing on shutting the engine on idling( at a traffic light) and not making engine run at close to zero RPM ( but certainly not zero) at idle speed by ECU?

So car engine keeps running and when the driver lift the feet from the "Gas paddle", the engine goes back to normal speed. In current scenario , when the engine shut down, the battery power is used and can put strain/Stress on battery.

I see cars has RPM meter that showsn Rotations per minutes in thousands, so, can't engine not run just at 100 RPM ( not in thousand) per minutes, drwaing very less GAS/Petrol)

I see others has similar concerns. And why not companies makes cars that "have engine that can stop just short of firing a cylinder"

You are right in the thinking of using less gas and that's exactly what the purpose was to have the start/stop feature. The main goal of doing it (at least here in the States) is for zero fuel consumption/zero emissions while the engine is stopped. This provides for better overall fuel consumption numbers to meet what the EPA has mandated for companies to stay in compliance with regulations. Considering what the government mandates, zero RPM will always produce zero emissions and have less fuel consumption than will 100RPM ... it all adds up.

WRT the start/stop causing wear/tear on the starter/battery which would otherwise not be there, you are accurate in thinking so. The manufacturer can do two things to help mitigate this that I can think of (whether they do them is another story):

1. They can design in more robust starting/battery system in the vehicle.


2. They can (and do) engineer the engine so it stops just short of firing a cylinder. When the engine goes to start again, it fires this piston to assist the starter in getting the engine started, thus helping to eliminate the wear/tear on the starting system.

Solar Mike is correct about V8's vs 3-cylinder engines and relative speeds to allow it to operate. This is less a function of the size (displacement or # of cylinders) of the engine, but more so to do with physics. An engine has to have enough rotating mass to continue motion (the old mass in motion tends to stay in motion kinda thing). The larger the rotating mass, the easier it is to keep things going. Secondary to that is the # of cylinders and how many cylinders fire per revolution of the crank. A V8 fires four cylinders every revolution (every 90° of crank rotation), while a 3-cylinder only fires a cylinder every 240° of crank rotation (given both are 4-stroke engines). The reason I point this out is with larger engines, such as those which are used in large cargo ships, run at full speed ~120RPM. Idle would be something much lower. Point is, the rotating mass of the engine allows it to run much lower in RPM. You can't fit a Wartsila engine in a car, though.

The engine needs a minimum rpm to idle smoothly - a v8 can idle smoothly about 500to 600rpm but a 1 litre 3 cylinder needs around 700 to 900rpm...

It is not the companies that really decided to do the start/stop, if you do some research the various governments have implemented legislation over the years that required drivers to turn off the engine when waiting at lights to reduce smog and fumes. The companies designed the start/stop systems because some drivers never bothered... it must be too difficult to turn the key sometimes...

Car companies focus on stop/start of the engine simply because

1. There's a minimum RPM the engine can run at. You can sort of extend it using dual inlet/exhaust variable valve timing, but there are limits to that.


2. If the engine is running, it's consuming some amount of fuel always.

Also, some companies go bit further than stop/start. They stop the engine when coasting, in a mild hybrid, typically using a 48-volt system.

Some companies go even further than mild hybrids, and have a full hybrid that can turn off the engine at significant speeds (up to 80 km/h or so), when the load demand is low, supplying the demand entirely from the battery.

The best hybrids have a separate electric coolant pump to supply cabin heat when engine is off, and an electric air conditioning compressor to cool the cabin when engine is off.

I'd say the future is clearly one of hybrids and electric cars. Your low RPM idea has no place in this future. Furthermore, at very low RPMs the oil pump might not be good enough because they are designed to operate at normal idling RPMs.