Sunday, July 13, 2008

MP2

As old Sam Clemens might have said you shouldn't read this post without you have read the one directly below so go do that and report back here stat. Okay then, onward. In this design exercise I will use as a notional base the approximate shape, size, and weight, of the aforementioned 1975 Scirocco. This size is large enough to be comfortable, sporty, utilitarian, and efficient in ways that tiny city micro cars can never be. Vehicles with near vestigial back seats such as the Scirocco are sometimes referred to as "two plus twos". This refers to the fact that although a car may have a back seat it is only meant for very occasional use by either children or very accomodating adults for short distances. Since this notional vehicle is intended as a second car for commuting or as a primary vehicle for a single or couple I propose eliminating the back seat altogether and incorporating the resulting space into flat load carrying area. Many vehicles have back seats that will fold down and extend the load floor but in this case the elimination of this seat will save as much as a hundred pounds of weight and allow it to carry an amount of cargo, if desired, that would rival a small SUV.

This seems like a trivial savings but, as any engineer knows, each additional pound of weight added to a car ripples through the design and construction of the rest of it. The goal in this case is to optimize the weight of all of the components to accomplish the task intended and no more except for small margins in the interests of service life and load carrying capability. Add a hundred pounds of back seat and associated attachments and that decision will race across the spreadsheet of the design like wildfire increasing slightly the size and weight of all the major components of a given vehicle. That hundred pounds can easily result in an overall weight gain of two or three times that much because the decision triggers a weight increasing feedback loop throughout the design. Additionally current vehicle designs incorporate much room for expansion into the engine/chassis/body architecture against contemplated future power increases which results in vehicles almost always weighing several hundred pounds more than strictly necessary. This phenomenon in concert with numerous weight increasing safety mandates is precisely the reason for the common 50% weight gain of vehicles in the last three decades.

The targets I have chosen for this notional vehicle are thus: An empty weight of less than 2000lbs with an engine that produces approximately 70 horsepower and 100lb/ft of torque in a front-engined rear drive configuration that seats two comfortably with a flat rear load floor contained in a low sleek longer than normal body style with two doors and a rear mounted hatchback style liftgate. Projected selling price in 2008 dollars would be in the 12 to 15 thousand dollar range. Mileage targets would be 50mpg highway and 35-40mpg city.

To begin this design exercise let's start with the engine. I see no reason why the engine in a sub 2000lb. car need be any larger in displacement than 1500cc. I also see no reason why this engine need develop any more than about 80HP and around 100lb.ft. of peak torque . This seems like a low bar by today's standards in which engines this size routinely develop a hundred plus horsepower but keeping power at a modest level would enhance efficiency and flatten the torque curve which makes for much more flexibility in surface street driving. Further I propose that as the vehicle were updated over the years that any such updates be in the realm of efficiency and not maximum power. A torquey eighty horsepower engine would push a slippery coupe to well over 100mph while its flat torque curve would minimize shifting while in town and provide plenty of oomph for situations such as passing slower traffic or entering freeway onramps.

The displacement of 1500cc is only suggested for it is entirely possible that modern engine design would permit the target power at a smaller size. This is undoubtedly true for the maximum power output but might be problematic in terms of the broad torque characteristics desired. A one liter engine could easily be tuned to deliver 70-80 horsepower, many motorcycle engines this size produce much more, but only with relatively high rotational speeds which could result in peaky power delivery, reduced service life, and poor operational flexibility in something much heavier than a motorcycle. Plus I'm skeptical that the torque target could be met by a powerplant that small without forced induction of some kind which further ups the ante in terms of the stronger/heavier internal parts required. It is not clear that a smaller much more highly tuned engine would deliver the same fuel economy as a larger engine which would be operating in a much more relaxed and flexible manner in daily driving. It is certainly possible that a lower level of horsepower and torque could deliver adequate performance, the 1975 Scirocco was significantly more than adequate performance wise, so efficiency might be enhanced without serious operational drawbacks by reducing power even further. An engine in the range of one liter of displacement, 61 cubic inches, would likely be fine if vehicle weight could be reduced below 1800lbs.

A four cylinder engine this size is inherently very smooth and would not require a heavy power-sapping balance shaft as do many larger fours and most V6 engines--a dozen or more pounds, and several horsepower, saved right there. Made mostly from aluminum alloy this engine should weigh no more than about two hundred pounds. Very reasonable and achievable today without the use of materials any more exotic than aluminum and steel. The rest of the engine can be as advanced as possible in today's terms incorporating high-pressure direct injection, low friction internals, and all the various tricks of the modern engine designer's trade. Contributing to the light weight of the engine is that internal parts such as pistons, connecting rods, crankshafts etc. should only be as strong as they need to be, plus a small safety margin, for the modest power level the engine would develop as well as the relatively low r.p.m. range involved. There would be no need to build unused strength into the engine's base structure and components as a hedge against future power increases in this design philosophy which I will admit seems anathema to virtually all manufacturers. Many current engine designs will allow huge increases in power without excessively stressing the internal components. This is understandable in vehicles that are marketed to the street tuner performance crowd but that approach is counterproductive in this design exercise.

This decision, a dramatic departure from current practice, would have several salutary effects. The rest of the drivetrain, the transmission and final drive assemblies, would need to be no stronger or heavier than necessary since engine output would increase little in subsequent model years. Why use a transmission design that can absorb two hundred lb/ft of torque when only around one hundred pounds will ever be generated? Why use a final drive assembly, or brakes, or wheels, or tires, or an overly stout body structure? Freezing the engine weight and power at modest levels will allow the rest of the vehicle to be designed with this carefully optimized approach which will be relative child's play for modern computer aided design systems. Limiting other operational parameters would further reduce weight.

A target braking distance of 120ft. from 60mph is a not unreasonable goal which allows more than adequate performance without requiring excessively heavy brake components. A target of about .75g of cornering force would allow crisp and sprightly handling without requiring a stiffly sprung suspension or wide wheels with rough riding low-profile tires. Ride comfort is a problem in small cars so achieving good handling without a harsh ride is definitely a bonus. There is no inherent reason whatsoever that a vehicle such as this cannot be pleasurable to drive, even fun, and building in a little driving sportiness is not contrary to the vehicle's primary missions of efficiency, utility, and cost-effectiveness.

As far as the look of this vehicle I propose dimensions similar to the notional Scirroco with a relatively low seating position, a longish nose which will increase crashworthiness and sufficient rear floor length to accomodate, say a couple of bicyles, and a carrying capacity of two to three hundred pounds of cargo. This would suffice for ninety plus percent plus of most people's daily needs. A lower longer body shape is far more aerodynamically efficient than the stubby upright style of today's small cars in general and micro-vehicles such as the Smart Car in particular. This is the most salient reason that they do not get the highway mileage that their small size appears to promise. As an illustration the relatively huge Ford Crown Victoria which weighs two tons can achieve close to 30mpg in careful highway driving. It is a large heavy car but it's aerodynamics are very good due to its length and slippery profile. In fact its inherent aero efficiency puts to shame the Smart Car. Emulating this longer sleeker profile to the maximum extent practical would pay off significantly in terms of highway mileage without greatly affecting city mileage.

With the use of wind tunnels and advanced computer simulations designing a sleek body should be easy if the design considerations encourage it. In this case a long nose, a low roof line and a longish properly truncated rear end will result in much enhanced efficiency as well as lower wind noise. Many people think that a low roof line involves both an uncomfortable seating position and is less conducive of the sort of traffic awareness that is touted so much in the world of the SUV. This is simply plain wrong in both cases. Comfortable seating is not that hard to design and proper adjustability, in the thigh support area especially, would eliminate any discomfort for either short or long trips. In terms of traffic awareness even the lowest sportscar is not deficient in this regard because the erratic, frequently curvilinear, nature of flowing traffic virtually always affords the drivers of even the lowest vehicles abundant situational awareness of what lies ahead. A Tahoe driver is not functionally able to anticipate traffic problems any better than a Miata driver. Even the smallest lowest vehicles I have ever driven seemed to have no trouble whatsoever in this regard and conversely sitting far higher and much more upright did not seem to confer any appreciable advantage. Indeed I am firmly in the camp that a light highly maneuverable vehicle will avoid most of the accident situations that a large clumsy SUV will simply blunder into due to its inferior braking and handling qualities. Given the choice between large unwieldy masses of damage absorbing steel and agile handling characteristics I am ever inclined to choose the latter.

It has become an engineering truism that front wheel drive (FWD) vehicles are inherently more efficient. I am not entirely convinced of this but it is true that FWD allows greater packaging efficiency in passenger vehicles. That is to say FWD will allow the maximum interior space for any given vehicle platform all else being equal. This increase in interior room is due largely to the lack of need of a driveshaft running through the center of the floorpan as required by rear wheel drive (RWD). Also some putative weight reductions are possible because the transmission and final drive assemblies can be contained in a single housing instead of the usual two for RWD.

There are downsides. Packaging all this equipment into the confined space of an engine compartment can be challenging and can make routine maintenance a genuine nightmare in many cases. An additional factor is that a flexible joint is needed at both ends of the twin driveshafts so that the engine can transmit power evenly to the front wheels which must be able to turn and move vertically. These CV joints as they are called are very smooth and efficient but they are much more complex and heavier than the simple universal joint found in typical RWD vehicles. Another issue is that a CV joint will only operate over a limited angular range with the consequence that the turning circle of the vehicle will be significantly greater than that of a normal RWD configuration. Lastly FWD is often viewed as a better choice in slippery weather conditions when a vehicle is driven by the typically inexpert driver. Modern anti-lock braking and traction control systems can greatly alleviate most of the handling problems associated with bad weather and rear wheel drive.

I cannot ignore the efficiencies of FWD but is my feeling that a more or less conventional rear wheel drive layout would be a good choice for our notional vehicle. When only needing to package two adults in front and with only a flat load floor behind then running a driveshaft through the floorpan is of little consequence. The driveshaft in such a low power application would be very compact, very light, and would use simple inexpensive universal joints which are wildly cheaper to replace than CV joints. The transmission would be relatively small in this application so it would not greatly intrude into the passenger compartment. Positioning the engine in a fore-aft configuration greatly simplifies under-hood packaging and maintenance and with no CV joints attached to the front wheels they can turn at much sharper angles which would reduce the turning circle and make manuevering into tight spaces easier.

Modern high performance vehicles tend to be RWD and often use an independent rear suspension (IRS) for the best possible handling. This solution works extremely well but requires a far higher parts count than the simple solid rear axle used in most American cars for a century. There is no inherent reason that a lightweight alloy solid axle could not be used for our project. An alloy unit designed to handle only the limited power output of this notional project could be very lightweight indeed and the parts count, and subsequent expense, would be dramatic lower than an IRS. We are considering a vehicle that would undoubtedly be fun to drive on twisty roads but would not intended as a canyon carver or a hot-lapper at the track. A simple solid axle suspended by two parallel composite springs would, if properly designed, result in a simple low cost reliable rear suspension. This sounds distinctly old school but if too much advanced (expensive) technology is used in the design then the price target could not reasonably be met. An independent rear suspension would certainly benefit handling and ride but its cost could be as much as a thousand dollars more than the notional alloy solid axle/composite leaf spring setup.

For this vehicle I propose to limit the transmission choices to one--a six speed manual. An automatic would not be offered and in any case would be contrary to efficiency goals. More exotic choices such as the Constant Velocity Transmission (CVT) can eke out a small increases in mileage but at the cost of far more complexity and consequent higher price. The standard flywheel/clutch/multi-speed transmission/driveshaft/solid rear axle configuration is about as mature and reliable a technology as exists today and it is conducive not only to low maintenance costs but also flexibility in traffic and, not inconsequentially, the fun of driving. The only question here is whether or not it should be a five or six speed manual. Six speed manual transmissions are becoming much more common and there is no reason not to take advantage of such hardware since it bears directly not only on efficiency but also on extracting the best performance from the limited power available. If designed specifically for the limited torque of the proposed powerplant the transmission, even with six speeds, could be very light, compact, and would shift very easily.

Moving on I suggest 14 inch alloy wheels with a tire in the 175/60/14 size. This would provide plenty of traction for spirited driving and good braking without increasing rolling resistance to unaceptable levels. No skinny over-inflated rubber doughnuts wanted here. Expensive low rolling resistance tires affect efficiency relatively little because resistance is it low at city speeds and at highway speeds it is inconsequential compared to air resistance. Very low aspect-ratio tires such as are found on high performance cars are not needful for this vehicle's mission and the greater air volume of the 60 series tires would contribute to ride quality with little sacrifice in traction. The front suspension could be made from lightweight alloy components with minimal cost impact and with less weight burdening the front tires the need for power-steering could be eliminated thereby reducing cost and weight enough to compensate somewhat for the expense of the suspension pieces. A front to rear weight balance of about 52%/48% would result in crisp handling without courting dangerous oversteer or invoking sluggish understeer.

If it's not clear already let me state definitively that I have no desire to suggest the automotive equivalent of a hair shirt or a puritanical device used primarily to burnish one's green credentials. If a vehicle is a mere appliance for ekeing out mileage then few, myself included, would be interested but this is a tradeoff that need not be made.

Since this notional vehicle would spend only a tiny fraction of the time anywhere near its top speed the body's aerodynamic design characteristics could be more optimized in the direction of low drag than is possible on many high performance vehicles. All the aerodynamic effluvia seen on cars such as air dams, rear wings, side skirting, etc help to keep a car from developing dangerous lift at high speeds but few of those add-ons do anything at legal speeds except increase drag. Modern aerodynamic design is perfectly capable of producing an attractive body style that has minimal drag with good highway manners if performance parameters are held in check as would be the case here.

In terms of interior appointments I of course have my own preferences but the styling motifs used would be largely irrelevant to the vehicle's core mission. Reducing the quantity of sound deadening material from current levels is recommended. The interior noise levels of small cars of 30 years ago were not all that onerous or tiring and it's my contention that a certain level of outside noise intrusion greatly increases a driver's situational awareness. I don't really consider airconditioning to be optional and it should be thoroughly integrated into the engine/body package. There is a fuel efficiency price to be paid of course but most people would prefer to at least have the option of using it in hot weather.

The mandated twin front airbags should be all that's needed here and in fact I recommend that the relevant mandates in any area should not be exceeded. Safety levels are high enough now. Besides if a car is designed for survivability in a 25mph frontal impact it does not become a catastrophic death trap at 26mph. In this case the longer than normal nose and distance between the occupants and the rear of the car will result in better inherent crash resistance than would otherwise be the case. The width of the car likely would need to be a couple of inches more than the Scirocco because let's face it the average American of today has been rather upsized from his 1975 counterpart. With the room available behind the seats long-legged folks should easily be accommodated despite the lower than normal roofline. Proper wheel and shifter positioning plus having the seats on inclined tracks should readily accommodate shorter drivers. An adjustable front seat bolster would give decent thigh support to the tall drink of water and a slightly flat-bottomed steering wheel would better accommodate the vertically challenged.

An effective free flowing ventilation system with several adjustable vents would reduce the need for air conditioning use substantially especially at speeds and temperatures that may not really need the AC to be on but would require the windows to be down for decent airflow. At 70 having the windows down sometimes increases drag enough to make the use of the air conditioner actually more efficient. This is particularly true in designs that are very highly optimized for low drag. Lowering the windows on a sleek optimized body shape will result in a larger percentage of drag increase than doing the same on something as inherently poor in this respect as a full size pickup.

It seems to me that minimizing the number (and weight) of all the electronic geegaws rampant on modern cars should be minimized. It will hardly inconvenience anyone to not have GPS navigation, or 400 watt six speaker sound systems, or DVD players, or electric mirrors, or rear view cameras, or blah blah blah. I'd recommend keeping the suite of electrical gimcrackery to a modest audio system with two speakers and decent instrumentation with a tachometer, speedometer, water temp, oil pressure, alternator, and fuel gauges. The buyer would be free to add whatever they deemed appropriate but for the standard configuration of the vehicle reducing the electrical load would mean that a smaller alternator and battery could be used. A smaller alternator would save a pound or three and reduce power sapping drag on the engine. It also might be practical, due to the single unit needed, that an advanced lithium type battery could be used to save possibly as much as twenty pounds.

The rumors of the death of the internal combustion engine (ICE) are premature, to put it mildly. This proposed concept would address the real needs of working-class folks far better than any of the pricey complex "alternative" vehicles out there. Making a vehicle such as this available would improve fleet mileage as much or more than the current hyrid crop that has a minumum buy-in of 25K. The key is optimization. If this, largely unused, approach were seen more often the gains in all classes of vehicle could be huge. The ICE has a lot of life left in it with this approach and using it is about the only way to build a true "peoples car" that is reasonably priced and more than adequately efficient. This philosophy should be able to be scaled up to almost any size vehicle. The temptation for manufacturers to overbuild is extremely strong however. That's the way they've done it, all of them, from time immemorial.

A frantic scramble to implement new technology is underway, in many cases prematurely, and inevitably those of us who won't be able to afford 30-40K for one of the flock of new whiz-bang hybrids or pure electrics will left out in the automotive cold. Self-styled progressives seem resolutely hung up on giving short or no shrift to "old" technology along with active opposition to increasing our oil supply which will have the effect of freezing out their supposedly beloved "workers" constituency from the green tea party. Perhaps we aren't all that beloved after all.

It will take a brave manufacturer to build such as has been suggested but if such were available right now they would be in extreme demand. With the unfortunately long lead times of vehicle design and manufacturing the soonest something like this can appear would be three years even with a hyperkinetic crash program. If the car companies can avoid being seduced entirely by the latest green chic there is hope. Not much it's true but it's there.

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