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ENERGY ) RECOVERY...

14th April 1978, Page 46
14th April 1978
Page 46
Page 47
Page 46, 14th April 1978 — ENERGY ) RECOVERY...
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Which of the following most accurately describes the problem?

BREAKTHROUGH OR PIPEDREAM

Regenerative braking — the recuperation of kinetic energy as opposed to its dissipation as heat — is a feature more easily realised in electric vehicles (battery or overhead powered) than in petrolor diesel-engined machines.

But the development engineer's ultimate goal of storing retardation energy for subsequent assistance in reaccelerating a vehicle is continuing to keep research workers busy in many parts of the world.

A few years ago (CM February 9 1973), we reported a Russian device in which a flywheel, mechanically revved up when the vehicle slowed down, was used to store retar

dation energy which could later be fed into the transmission.

Although vehicles powered by an on-board source of stored physical energy, such as a flywheel, have made intermittent appearances since the first Gyrobus was completed in Switzerland over 27 years ago, the potential of absorbing retardation forces in the inertial energy source has only been explored more recently.

A demonstration, a short while ago, of a petrol-engined American Ford Pinto car converted to a flywheel transmission-retardation system was followed last month by the presentation of a prototype mail van (Garrett AiResearch Corp) for the US Post Office, in which the battery electric drive-line similarly incorporates a flywheel charged with the dual function of optimising driving and braking.

But a conference last November at the Lawrence Livermore Laboratory, the view was widely held that for at least ten years the flywheel as an energy source would have no more than an "R and D" role.

In the mid-Seventies an "energy regenerating retarder system" was proposed by an independent group of engineers for application to a Leyland National bus. In this scheme, which was carefully considered by London Transport but pigeonholed 18 months ago, a transfer box interposed in the conventional drive line was coupled to a reversible hydrostatic pump/ motor so that, on braking, retardation energy would charge up a system of high and low press

ure reservoirs in which tl hydraulic medium would I stored against a compressiL gas cushion.

Now that London Transport experimenting with the prof type of an hydrostatic transm sion system designed I the National Engineerir Laboratory, the option of an e ergy storage device again arise In the meantime, Germz. researchers have taken adva tage of the financial backing 1 the Bonn Government to put ( the road a fully operational pr totype of a hydrostatic prop( sion system. Almost the san principle is used as propos( earlier in the UK, and gives ft., savings of up to 20 per cent.

The German programme, i itiated in November 1975 by team of engineers from the I stitute of Automotive E gineering, Berlin University, tl Berlin Transport Authori (BVG) and "Berliner Verkeh Consulting", an affiliated trar port consultancy group, has al had considerable technical su port from specialist supplie such as Rexroth and Hydac.

The primary aim of the iearch project — the cornited prototype is based on a 164 Kissing underfloorgined vehicle — is to establish how much fuel consumption

be reduced and to test the lability, efficiency and safety the propulsion system. So far ?I savings have already exeded the target figure len compared to a conventio1 city bus of similar weight d engine ratings, running der identical conditions.

As fuel costs in the total aakdown of city bus operating sts do not represent a )nificant proportion — esdally when tax rebates are ten into account — this saving elf is not of great conseence. But it is enough at least balance out the additional restment cost for the extra ive line/ storage equipment er the-'normal life span of the hide.

In addition to the albeit slight ,erating economy, the advanges of the hybrid system are: reduction of exhaust emisIn in scale with the increased ?I economy

marked reduction of the noise d exhaust nuisance to ssengers at bus stops due to 3 "hydraulic drive" mode len pulling away

optimum acceleration cha3eristics and an improvement ride comfort inherent in steps acceleration and deceleran.

The parameters of city bus eration, with its constant ;p-go performance cycle, set a od basis for the application of -egenerative propulsion sys*n. In the Berlin project,

draulic accumulator iciency is claimed to be of the ler of 90 per cent uSing a high formance pump/ motor unit for driving and deceleration.

In its present experimental phase, the Berlin test bus has retained its fully automatic Voith Diwabus transmission to provide for a conventional operating mode for purposes of comparison. In a vehicle designed from scratch to incorporate a hydrostatic propulsion system, there should, of course, be no need for a fluid flywheel or mechanical transmission, and the installed engine rating can also be lower.

Because the original 68 kW (91 bhp) engine and Diwabus unit have been retained and because of the additional drive line components (supplementary transfer box, hydrostatic pump/ motor and accumulator tanks), the unladen weight of the bus has increased from 6.7 to 8.3 tonnes. Ballast weights have also been installed to compensate for the left-sided arrangement of the steel hydraulic tanks, the only suitable location in the vehicle. The weight penalty — which can be assumed to simulate a part payload — could be reduced to no more than 300 kg (6 cwt) in a purpose-built hydrostatic drive vehicle

Of particular interest in the Berlin experiment is the automation of control gear to relieve the driver of the need to select the required drive line mode In the hybrid mode the command from the accelerator. is fed to the hydrostatic pump/ motor. Only when there is no —energy" in the storage accumulator will the black box call for increased power from the diesel engine.

Use of the brake pedal similarly controls the hydrostatic pump/ motor as well as the constant displacement hydraulic pump_ Recuperative braking is wholly dependent on the state of charge (pressure) in the accu mulator. After a certain pedal travel, conventional mechanical braking is introduced.

Another feature is the automatic pressurising of the accumulator while a bus is waiting at a stop. This too is obtained without intervention by the driver..._ The accompanying diagram illustrates the four main modes in which hydrostatic drive or retardation is used. Although the nominal rating of the installed engine is 68 kW, in combination with the hydrosystem a short term acceleration rating of 140 kW (188 bhp) is available. The hydraulic pressure vessels are designed to operate at 28,300 kn/sqm (4,100 psi) max and have a iiominal volume of 232 litres (51 gal). Operating speed in the hybrid mode — diesel hydraulic — is 50 km/h (31 mph).


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