Economical engines

Posted on October 30th, 2005 in Opinion by Julian Edgar

Start talking the fuel economy of different petrol engine types and designs and things get complex, fast. Amongst other factors, fuel economy is affected by internal friction, pumping losses, combustion inefficiencies and the air/fuel ratio that is used.

The biggie with internal friction is, literally, how big the engine is. A larger engine has longer internal bits rubbing other bits, and so a 5.7 litre V8 is always going to have poorer fuel consumption that a 2-litre four cylinder. That statement applies when both engines are producing the same low power required for cruising, but may not be the case when the power demand is high – climbing a hill while pulling a trailer, for example. In the latter situation, the smaller engine will have to rev very hard to develop adequate power, and the higher the speed of the engine, the greater the power loss through friction. Because of the high loads to which it is being subjected, the small engine might also move out of closed loop (ie ~14.7:1 air/fuel ratio) to a much richer mixture. So as the power demand increases, the practical on-road fuel consumption may not so clearly favour the smaller engine over the larger engine.

Another way of seeing this is to look at the fuel economy gained from a small engine car that always has to have the ring driven out of it to keep up with traffic. In this situation, the fuel economy is often poorer than the larger engine car that is always just loafing along.

Pumping losses refer to the drag caused on the movement of the pistons on their intake and exhaust strokes. Any restriction on the intake – including, critically, the partly closed throttle – will lower the pressure of the air that fills the cylinder on the intake stroke. Rather like drawing down a syringe that has the needle opening blocked, power is needed to overcome this partial vacuum. On the exhaust stroke, anything that restricts flow out of the cylinder – from a poorly flowing muffler to bad port design – will again require power that’s subtracted from what is available at the flywheel.

Some BMW engines dispense with the throttle – and instead change intake flows by varying valve lift and timing – however the intake pumping losses remain. (Diesel engines, of course have no throttle and so much smaller pumping losses.) A better approach to reducing pumping losses is to adopt the Atkinson or Miller cycles, where the closing of the intake valves is much delayed at lower engine rpm. This poorer intake flow requires the driver to more widely open the throttle for a given power output, so reducing pumping losses. However, the engine also develops less power because its volumetric efficiency is much lower than an engine with conventional valve timing. Atkinson/Miller cycles are therefore used only when there is forced induction at low revs (eg a supercharger as in the Eunos 800M) or power is available from an electric motor (Toyoya Prius and most other current hybrids). 

Oldies but goodies

Posted on October 23rd, 2005 in Opinion by Julian Edgar

I love reading; in fact it puzzles me slightly that anyone who has any interest in anything wouldn’t love reading. I also love buying secondhand goods; put those two together and lots of old books come my way. Via eBay, from garage sales and secondhand book shops, at auctions and by tender.

Over the last year or two I have been buying lots of old car books, especially those that deal with car technology. Given that my major car modification interests are electronic systems, turbocharging, aerodynamics and hybrid cars, you might wonder why I’d bother buying old car engineering books. After all, aren’t they all way outdated?

Well, yes and no.

Sure, you won’t find mention of the latest in Bosch electronic stability controls, or active aero, or ball-bearing turbos. But equally, there’s been almost zero change in car fundamentals. Engines still have pistons and cams and crankshafts, the concept of valve timing hasn’t changed much in 100 years, and Ackerman steering geometries have as much validity then as now. Even more importantly, the physics of power and torque and engine revs; sprung and unsprung weight; engine balance – and a host of other topics – hasn’t changed one whit.

And the best thing about some of these books is that the way they explain these concepts is simply second to none. Perhaps in times past it was much more common for someone to get interested in a topic, buy some books and then set out to teach themself the whole thing from scratch. (These days, the same person just subscribes to a web discussion group and gets a mix of advice that is typically 80:20 in quality… and no, that’s not 80 per cent good stuff!)

The weight changes caused by turbocharging and supercharging

Posted on October 9th, 2005 in Opinion by Julian Edgar

Many performance mods add weight to a car, so reducing their real world effectiveness by at least a little. A big exhaust, a supercharger or turbo – all make the car heavier than it was standard. (Of course there are a couple of mods that make the car a tiny bit lighter – eg porting a cylinder head or lightening a flywheel – and there are other modifications that make no difference at all to car weight – eg increasing turbo boost.) But in general, even changes like bigger wheels add mass.

Recently I had the opportunity of comparing the weight gains made to a naturally aspirated car that was firstly supercharged, and then the supercharger removed and the engine instead turbocharged. How much weight was gained by each approach? The car was my first series Toyota Prius.

The blower installation involved the fitment of: