Motoring money saving tips

Herbert_Stern

Motoring money saving tips

Unfortunately, these tips contain at least one glaring and common error:
The sort of percentage fuel savings quoted by weight reduction apply to the reduction in rolling resistance. The rolling resistance however typically amounts to only about 20% of the total drag, so that the fuel savings claimed by you due to vehicle load weight reduction have to be divided by 5. This makes it quite uneconomical to make the extra trips to filling stations that the recommended filling of the fuel tank to only 50% of capacity entails.
I can confirm this by personal experience, as I used to transport 84 two litre bottles of good Edinburgh water to my 400 mile distant Wembley home, i.e. about a 15% increase in vehicle all up weight, without any really noticeable increase in fuel consumption (I usually filled the fuel tank fully at both the start and end of the journey and divided the fuel quantity by the distance covered).

Your advice on fuel saving by gentle acceleration to the desired speed is also somewhat suspect, at least in the case of petrol engines. , due to their poor part load efficiency arising from the energy loss imposed by the partially closed throttle. The amount of kinetic energy imparted to the vehicle to bring it to any given speed is, of course, the same and irrespective of the rate of acceleration. In town, on my Diesel engine car, I start in first gear, quickly change into second gear, which brings the car up to the desired speed in a very few seconds, and then change into 4th gear, thus saving a gear change and cutting down the time the engine consumes fuel in neutral.

Your reference to AA compiled motoring costs of £ 3000 p.a. is also misleading. This figure appears to have been plucked from the AA motoring costs 2014 (available on the internet) for petrol engine cars in the £ 13,000 to 18,000 price category and only purports to represent the standing charges, i.e. the running costs per mile still have to be added.
These standing charges have been compiled on the assumption that people buy a new car every 4 years, resulting in a quite unnecessarily high depreciation figure, except for those people who drive extraordinarily high mileages per year. Modern cars will last mechanically without unduly high maintenance costs for well over 100,000 miles and even the less durable makes last 10 years without rust problems. The AA seems to be more than half a century behind in time, when white metal crankshaft bearings were indeed worn out at 50,000 miles, engine oils were less advanced thus requiring frequent decokes, suspension joints were not sealed and leaf springs were the rule for the rear suspension.
There is no justification whatever for buying a new car every 4 years, unless unusually high annual mileages are covered. The earliest a change of car could possibly be considered is just before the camshaft drive belt is due for renewal at 60,000 miles. Otherwise a good modern car will last for much more than twice that mileage.
An entirely good, moderate mileage used car about 3 years old can be bought from a car supermarket or privately for about 60%.of the cost of a new car, thus reducing the depreciation cost even further.
The AA running costs are also somewhat excessive.
4 new tyres are not needed in 4 years for the annual mileages covered by most people.
Servicing by expensive main dealers is also questionable. I will only mention 2 of the horror stories I could tell about them, namely where the car owners were fraudulently advised that they needed a replacement engine.
In one case a friend of a friend took her only 25,000 mile Metro car to an Austin/Rover dealer for routine serving, whereupon she was told that there was a knock from the engine bearings and that she needed a reconditioned engine. She enlisted my help and we went to the dealer unannounced. There I started to examine the bearings. The first big end bearing looked unscathed and the bearing clearance was well within the specified range. I was about to inspect the second big end bearing, when the service manager emerged from his office and said “There has been a mistake, there is nothing wrong with the engine”.
Secondly a friend of mine found that the engine of her Ford Escort was not running smoothly. I diagnosed loss of compression in one cylinder and she took the car to a prominent Ford main dealer. She, too, was told that she needed a reconditioned engine. We then visited the dealership, where the mechanic rocked the piston in the affected cylinder backward and forwards longitudinally to demonstrate that the cylinder bore was badly worn. There is, of course, a considerable clearance when the parts are cold, because of the difference in the coefficients of thermal expansion between the aluminium piston and the cast iron cylinder. In any case, cylinder bore wear takes place in the transverse direction, due to the inclination of the connecting rod. All that was needed was the removal of a local carbon deposit on the seating surface of one of the inlet valves. This was either a case of incredible incompetence or yet another scam based on the assumed technical ignorance of the car owner.

Herbert Stern
Chartered Mechanical Engineer

pogofish

Are you posting in a professional capacity without clrarance from MSE?

reeac

I've always assumed that the AA reported figures for car costs were produced to justify excessive mileage rates for business users.

BeenThroughItAll

I'd question whether someone whose posting relies on examples plucked from a time when Austin Rover dealers were still in existence is as expert as they think they are - although I agree with some of the points raised, there is a deeply suspicious aura around the post.

colino

With your style of writing, your either DM yet again, or is your first name Glacier?

ballyblack

:rotfl:Whats a 'metro car' and a 'Ford Escort' ???

WellKnownSid

Your analysis of the petrol engine is about two decades out of date.

Whilst pumping losses in the Otto cycle can be significant under partial load - the reality is that manufacturers have been addressing this for years. Exhaust Gas Recirculation, variable valve timing, electronic valve timing, GDI engines, and a variety of throttle modifications including throttle-less operation have been in use over many years. The reduction in friction losses through losing a cylinder coupled with a light pressure turbo have further improved the picture.

Gentle acceleration is obviously always best. Why? Because after accelerating quickly, even assuming no additional losses in the transmission system and tyres, you'll spend a longer at a higher speed, where air friction is proportional to velocity cubed. Furthermore, you'll then catch up with traffic in front where you brake, and unless you drive a hybrid - 100% of that kinetic energy will be converted into heat in the friction material of your braking system.

That's the trouble with MechE.'s. They're never in the real world.

Well Known Sid.
Chartered Electrical Engineer.

reeac

Catching up with slower traffic and then braking is hardly good MSE practice although, goodness knows, one sees plenty of that kind of driving going on.

Car_54

WellKnownSid wrote: »

Gentle acceleration is obviously always best. Why? Because after accelerating quickly, even assuming no additional losses in the transmission system and tyres, you'll spend a longer at a higher speed, where air friction is proportional to velocity cubed. Furthermore, you'll then catch up with traffic in front where you brake, and unless you drive a hybrid - 100% of that kinetic energy will be converted into heat in the friction material of your braking system.

When I was in school it was only velocity squared. Is that inflation? Should we blame the coalition government?

straighttalker

The biggest cost of motoring which most people ignore is depreciation. I'm getting sick to the teeth of people chopping their car in for one which costs "only £20 a year to tax" or "does 60mpg". What they seem to forget is the many thousands of pounds that they lost on selling their old car to get into their nearly new econo sh!tbox.

Mobeer

Car_54 wrote: »

When I was in school it was only velocity squared. Is that inflation? Should we blame the coalition government?

Not sure about assigning blame, but correct on the maths - its the square of velocity. But then energy needed to prevent the car slowing down is related to the drag times the velocity, which seems to be what the writer meant.

Gentle acceleration is best for economy because you should drive slowly. Very slowly.