Good mechanical design work is a blend of form, fit, function, analysis, materials choice, manufacturing efficiency, and technology balance to name just a few. It is not a simple process by any means and can be difficult to do well. It all begins with a need.
Necessity and Invention form the beginning and end of the design process. Necessity being the problem to be solved, and invention being the final result of the design process.
The designer is faced with a difficult task of trying to balance all of the needs involved in making a product cost effective, easily manufactured, durable (this assuming that the product is not single use), and, of course, solves the problem it was designed to solve. Many of the main features of the design are going to conflict with one another.
Automobile design is a good example. Back in the mid-1980s, I owned a Honda Civic. This was a two-door hatchback with a 1.5 L engine equipped with a carburetor and no air conditioning. This without much effort at conservation, I could achieve over 40 miles per gallon. During the same time period, Volkswagen produced a diesel engine equipped passenger car that could achieve 50 miles per gallon, or better. Currently, my 2000 Honda Civic with a slightly larger engine, fuel injected, and considerably heavier, will only achieve 40 miles per gallon on long stretches of freeway driving. My 1986 Civic hatchback had an empty weight of around 1700 pounds. My current four-door Civic has an empty weight around 2500 pounds. The newer vehicle does have a slightly larger engine at 1.6 L and much higher power output, being rated at over 110 hp as opposed to the old Civic power rating of around 72 hp. The newer car is much safer, and much more comfortable. The trade-off in efficiency, in my opinion, is a small price to pay for the added creature comforts and safety of a larger, heavier vehicle.
Yet, could the added safety and comfort of the larger, heavier vehicle be had by using lighter materials and a more advanced structural design? Yes, of course it could. The problem this creates is higher cost. The best materials from the weight-strength perspective are composites. They also have the great advantage of durability concerning exposure to road salt. Now, we may have a vehicle that could achieve better than 50 miles per gallon with current engine and transmission technology, but at a considerably higher cost. How much higher I can only speculate. The increased costs could easily drive the retail price to a point where market share would be drastically reduced.
It is also possible that a compromise material could be chosen, like aluminum. This material is one third the weight of steel, and can be manufactured with strengths equal to or better than many grades of steel used in automotive design. It is also very corrosion resistant. Aluminum can also be recycled very easily, as compared to composite materials, where recycling may be difficult or impossible to achieve economically.
Lighter weight has many advantages. With less mass to accelerate, a smaller engine could be used. A smaller engine will weigh less. The smaller engine also consumes less fuel, meaning a smaller gas tank, smaller engine mounts, a smaller and lighter transmission and other powertrain components, which leads to more weight reduction. A smaller engine requires a smaller engine bay, a smaller hood, a smaller and lighter exhaust system. A lighter weight vehicle will also require smaller suspension components, and lighter weight tires, which means lighter weight wheel rims. All of this could result in an even smaller engine. And so the cycle repeats. All of these material reductions will reduce manufacturing costs, improving its profitability and market share.
A diesel engine could also be used with current body designs and current materials to achieve the same or better efficiency. Case in point, the Euro Civic has a 1.6 L diesel engine and achieves over 60 MPG. These are not sold in the USA, which is unfortunate as the great improvement in efficiency is the only reason I would consider buying a new car. Now, if this car were lighter… 70 mpg?
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org