The complex factors driving the fuel-efficiency stagnation paradox
By Dr. Mathias Woydt and Dr. Raj Shah
The past century has seen countless innovations within the automotive industry making cars and trucks more reliable, powerful, comfortable, and less expensive. Despite all of these improvements to the design and manufacture of automobiles, average fuel economy has seemingly stagnated.
Cars like the antiquated Ford Model T were reportedly able to achieve up to 23 miles per gallon (mpg) or 10.2 L/100 km. In 1934, Continental Automobile Company advertised their “De Luxe 4-Door Sedan” which was equipped with their 40 HP inline 4 engine capable of achieving a claimed 25-30 mpg (Figure 1). This is consistent with the federal government’s fuel economy rating of a combined 26 mpg (9.05L/100 km). The average fuel economy of a new car in 2018 was 25.1 mpg (9.37L/100 km or 353 grams CO2/mile) with an increase to 25.5 mpg (9.22 L/100 km) for 2019.
Based on this stagnation in fuel economy how do automakers expect to achieve the fuel economy ratings set forth by the Corporate Average Fuel Economy (CAFE) regulation of 2011? The CAFE regulation sets forth an average fuel economy target of 54.5 mpg (4.31 L/100 km) for 2026 models (Figure 1). An analysis by the U.S. EPA in 2016 adjusted this number to 50.8 mpg (4.63 L/100 km) based on updated projections.
This CAFE regulation was revised on 31st March 2020 as the Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule and sets a lower target of 1.5% yearly increase through 2026.This will lead to a projected 40.5 mpg (5.81 L/100 km) or 201 grams of CO2 per mile (124.8 g CO2/km) by model year 2030.
At first glance it seems that in 100 years no significant progress occurred, but this is not the case. Many innovations that would increase the fuel economy of cars such as advances in fuel and lubricant additives, engine efficiency, advancements in metallurgy, and vehicle body design have been offset by additions to the car elsewhere such as increased crash safety, amenities and comfort features, and engine performance. Compression ratios in early gasoline cars with 4-5 bars were very low compared to modern vehicles with 11-13 bars due in part to engine knock at higher compression levels. In order to reduce knock, fuel additives such as tetraethyl lead, alcohols, ethers and aromatics were developed to increase octane rating. The development of these octane boosting additives allow modern vehicles to be designed with larger compression ratios when compared to early gasoline cars.
Another factor to consider is the increase in weight, size and front area of modern cars. The Ford Mustang has increased in weight by nearly 1,000 pounds (454 kg) from 1965 to 2020 with much of the weight going towards safety equipment such as airbags, chassis components, anti-lock braking system (ABS), and crash mitigation systems. The curb weight of the first generation Volkswagen Golf GTI was 1,834 lbs (832 kgs), which has ballooned to more than 3,000 lbs (1360 kg) in the seventh generation of the Golf GTI. An increase in vehicle weight will reduce the vehicle’s fuel efficiency ratings. At the same time, the power of the gasoline engine increased from 110 HP to 230HP.
From 1980-2018, the corresponding efficiency tradeoffs amount to 24 g CO2/km (38.6 g of CO2 per mile) for gasoline cars and 40 g CO2/km (64.4 g of CO2 per mile) for diesel cars. However, if the curb weights, horsepower, and front areas remained the same as that of cars built in the 1980’s, then the CO2 emissions of gasoline and diesel models could be 13% and 25% lower respectively. The impact of vehicle attributes on the CO2 emissions of compact cars (VW Golf, Ford Escort, GM Opel Astra) from 1980-2018 is displayed in Table 1 based on a study by the European Commission’s Joint Research Center in Ispra, Italy. Additional weight is also added by the multitude of electrical features and passive safety equipment that exists in new cars.
Lubricants have also improved in the past century leading to significant improvements in engine efficiency and reliability. The engine oil ratings have evolved from the 1930’s API SB to today´s API SN Plus and above. In this timeline, the pool viscosity went from monograde SAE 50 (API SB) to multigrade SAE 0W-20/0W-16 (API SN), which yields a reduction of the kinematic viscosity of the oil by between 60% to 70% at 100°C. In general, lower viscosity oils reduce engine friction and improve the fuel economy, however, the effects are smaller than originally thought. The viscometric impact by switching from one SAE viscosity grade to another, assuming all other variables are equal, improves fuel economy by less than ~1%.
Summarizing the literature from the last two decades, the fuel economy improvement (FEI) ranges between 1.6 to 2.3% FEI per 1 mPas of high temperature high shear viscosity (HTHS). HTHS denotes the high temperature shear viscosity at 150°C and a shear rate of 106 s-1. The risk of reducing oil viscosity may result in accelerated wear because of reduced oil film thickness which has to be combated by improvements in metallurgy, coatings and additives.
Friction Modifiers (FM) reduce friction coefficients in tribosystems under the mixed/boundary lubrication regime, whereas viscometrics affect the hydrodynamic fluid film formations and friction in this lubrication regime. Organomolybdenum-based friction modifiers have seen a rise in popularity in modern engine oil formulations since they are multi-functional and can achieve an increase of around 1% to fuel economy. In particular, molybdenum dialkyl dithiocarbamates (Mo-dtc, CAS: 71342-89-7) are seen as crucial and effective FM additives in the automobile industry. Organic friction modifiers have also proven effective in fresh engine oils. The retention of friction reduction between drain intervals and the interaction with metallurgy and coatings are the future tasks of additive-based concepts.
Metallurgical solutions, such as diamond-like carbon (DLC) thin films, offer a reduction in friction as long as the film doesn’t wear away or break. DLC, chromium nitride (CrNx) and molybdenum nitride (MoNx) films can achieve fuel economy increases of between 1-2% when tested in different tribosystems of the engine.
To increase the effectiveness of metallurgically based thin film coatings, specific additives will need to be mixed in. These additives include glycerol monooleate (GMO, 111-03-5) and glycerol monostearate (66085-00-5). These combinations can lead to specific oil formulations with limited miscibility and interchangeability with other grades.
Forced induction smaller displacement or “downsized” engines have become more common in modern cars compared to the large displacement naturally aspirated engines typical in older cars. Today’s gasoline engines have reached 150 HP/L and diesel engines exceed 100 HP/L. The inline 3/4 cylinder has become the most popular engine configuration, overtaking V6 and V8 engine configurations. This is due in part to the introduction of reliable turbocharging which can allow a smaller engine to achieve the torque and power numbers of a larger engine while allowing for the higher fuel efficiency ratings of smaller engines. The rationale behind “downsizing” is to have less cylinders and thus less friction, but with the same or higher power density. The added strain on the engine increased the demand for high performance lubricants which can protect against oxidation, scuffing and low speed pre-ignition. These issues are compounded with the lower viscosity oils that Japanese automakers introduced and are now more commonplace.
Further improvements in fuel economy through reduced engine friction are expected with the arrival of alternative engine oils based on ester and polyglycol formulations with viscosity indices (VI) above 200 and ultralow viscosity grades (e.g. SAE 8 or 12). Furthermore, octane ratings above 100 will also result in better fuel economy due to higher compression ratios.
There are many factors to consider when assessing the fuel efficiency of modern vehicles. Although gas mileage appears to have stagnated since the early gasoline vehicles, modern vehicles offer better engine performance, active and passive safety, and other quality of life features that older vehicles did not offer. In some cases, the introduction of these advanced features placed additional stress on the engine, resulting in lower fuel efficiency. The development of more effective fuel and lubricant additives in tandem with advancements in engine performance, have compensated for this, allowing for further improvements in the fuel efficiency of the internal combustion engine.