Evaluating School Bus Fuel Systems

Never before has there been more choice or debate on school bus power systems. Gas, propane, diesel and electric options all have advantages and disadvantages. It’s important to evaluate them on the latest technologies rather than old rationales.

2019 Federal Emissions Report

Diesel powered buses, which are the most common buses are used as the baseline from which other platforms operating and capital costs are measured. The rise in propane and gasoline power system popularity is largely a result of customer frustration with first- and second-generation diesel emissions systems that reduced reliability and increased maintenance costs. Does this still hold true?

There are currently 4 fuel choices. Each have very different up-front capital and operating costs which, when combined, give the total cost of ownership (TCO). Below is a list of each power system based on fuel type.


Diesel power systems have evolved immensely in recent years. The third-generation emissions systems of medium duty diesel power have been on the North American market for about 18 months and shows vast improvements in reliability, cleanliness, and fuel economy over first- and second-generation diesel emission systems.

Detroit DD5

  • The most modern diesel 3RD generation emissions platform
  • Lowest fuel consumption of any fossil fuel platform
  • Designed from the ground up to meet todays and upcoming emissions standards
  • Extensive durability testing – operating in Europe for 6 years
  • Extendable emissions warranty
  • Lowest overall emissions. Lowest CO2 production of any platform except electric
  • Fewer regens and lower DEF consumption
  • Simpler, more robust Turbo Charger for longer life
  • Self-Cleaning EGR. Exclusive variable cam phasing to reduce after treatment
  • Annual oil change intervals
  • Designed to operate in Vocational (Urban) applications
  • Virtual Technician – live engine diagnostics while on route
  • Mid-level cost to purchase plus lowest fuel consumption and advanced design to reduce maintenance put it at the low end of the TCO


  • Second generation emissions design
  • Low fuel consumption but still about 7 to 10% higher than the Detroit DD5
  • Historic engine that was adapted to meet current emissions requirements
  • Higher maintenance required on the emissions system due to a lack of ground up design
  • Variable Vane Turbo Charger that is higher maintenance with typically about half the life of the engine
  • EGR prone to plugging
  • Usually 2 engine oil services per year
  • Common parts
  • Operates better on rural than urban routes as the Cummins emissions system design has its challenges in urban applications
  • Has good range
  • Medium capital cost
  • Overall TCO somewhat higher due to emissions and turbo maintenance on an old design

Maxxforce Navistar V8

  • Out of production but still common in many fleets
  • Highest fuel consumption of any diesel and very poor reliability with high emissions
  • First generation emissions system using poor design that unfortunately gave the once reliable diesel engine platform a poor reputation
  • Low purchase cost at the time and sold in volume based on price
  • Very high TCO because of engine failures
  • A good example of how not to build a diesel engine


There are currently 2 engine choices available on the market. Propane fuel systems are a simple, reliable engine platform. They are clean running but do have a significant increase in CO2 output due to increased fuel consumption. Cost analysis on operating propane buses is often based on the assumption that propane will always be available at a massive discount to make up for the doubling of fuel consumption per km. It’s also assumed that propane engines will last as long as well-designed diesel, which historically is not the case. Statistics bear out that in industrial high-hour applications, propane engines last about half the life of a diesel engine. Long term life in school bus application is yet unproven.

When considering fuel costs, future developments may affect TCO. 4 export terminals planned for the BC coast and 2 large industrial complexes completing in Alberta in the next few years will greatly increase demand for propane and likely reduce the current surplus resulting in rising prices for the fuel. Anticipated future increases in fuel costs, coupled with very high fuel consumption, higher initial capital cost to purchase and unproven long-term durability can easily erase anticipated savings of propane power systems.

  • Highest fuel consumption
  • Higher capital cost – can be $12,000 more to purchase than a diesel school bus
  • Double the fuel consumption of diesel and about 15% higher than gasoline
  • Short range and increased fueling time
  • Expensive in-tank fuel pump replacements 2-3 times over the life of the bus
  • Higher fuel volatility can be a significant safety hazard in the shop and on the road
  • More affordable engine platform to replace than diesel
  • Three to four engine oil services per year
  • Historically propane can be the cheapest fuel per litre but this is uncertain going forward as industrial demand looks to increase sharply in the next 5 years reducing the surplus
  • Propane has limited range of travel; availability of fueling stations during extended travel is a concern

PSI HD 8.8 L

  • Uses a GM big block and proven HD Allison transmissions
  • Low rev high torque design
  • Used by Thomas Built Buses and Freightliner as well as IC Bus


  • High rev engine with smaller displacement
  • Uses Ford Transmissions
  • Borrowed from Fords HD pickup truck platforms
  • Used by Blue Bird Bus


In decades past, gasoline was the only choice in school buses. When reliable, affordable diesels came to market, the gasoline school bus quickly disappeared due to the significant savings in fuel, about half the consumption. Gas has made a comeback largely as a response to reliability issues in early generation diesel emissions systems. More recently, there has been some misrepresentation on the actual fuel economy of today’s gasoline buses. While slightly better than years ago, they still use almost double the fuel of a modern diesel, which greatly increases TCO.

  • Available in PSI 8.8L GM or Ford 6.8l Ford Roush
  • Lowest cost to purchase
  • Same base engine as propane engine
  • Usually about $5,000 less to purchase than diesel
  • Simple and reliable
  • Three to four engine oil services per year
  • Higher CO2 output from high fuel consumption
  • Shorter range requiring more fueling stops
  • Almost double the fuel consumption of diesel
  • Low maintenance costs
  • Common fuel available everywhere
  • Higher volatility fuel that can compromise safety
  • Higher TCO largely from very high fuel consumption and fuel cost that on average is 90% of the diesel fuel cost per litre
  • Over 300,000km life of the bus it can easily burn $65,000 more in fuel than a diesel engine or about 2/3 the initial capital cost of the bus in extra fuel
  • The best application for this platform is in remote areas on short runs
  • New emissions requirements will likely drive up the capital cost to purchase this bus in coming years
  • Shorter engine life compared to a well-designed diesel


Electric vehicles in general are getting a lot of interest; advancements in school bus power systems are reflective of this. Electric school buses work well but require massive subsidies to purchase.  They also require expensive infrastructure, such as charging stations. The TCO is generally unattainable for most school board budgets that are already stretched thin. This emerging technology holds promise, given time, but could be ten years before it is cost effective without large subsidies.  Usually still needs fossil fuels burned to generate the required volumes of electricity to charge.

  • Currently extremely high cost to purchase: at least 3 times the cost of a propane, diesel or gasoline bus
  • $100,000 batteries usually last half the life of the bus
  • Very reliable and low maintenance and fuel costs
  • Quite clean
  • Expensive infrastructure at a central location to charge buses in 3 to 6 hours
  • Shortest range
  • Alberta climate necessitates a large diesel fired coolant heating system to keep the interior and batteries at optimum temperature during the winter months
  • Currently an evolving technology that is well out of reach for most customers to afford without subsidies
  • Not recommended outside urban applications
  • Highest TCO due to purchase price, battery replacement and infrastructure costs


Customer preferences play a large roll in fuel choices. Rather than considering true operating costs, some may choose a higher TCO power system because it’s what they are familiar with, or they anticipate propane and gasoline may provide fuel or maintenance savings over earlier design diesel emissions systems. There is an abundance of misinformation promoting propane and gasoline over diesel as the greenest and lowest TCO. In fact, the long-term durability of propane and gasoline platforms is unproven in school buses. Much of this data compares propane to older diesels with first- and second-generation emissions systems that had lower fuel economy and higher maintenance. This is not true with the latest generation diesel.

The latest third generation of diesels is similar to the evolution from a propeller to jet aircraft. Progress brings advancements in efficiency, cleanliness and reliability thus reducing the TCO to operate. Currently in the school bus industry, only Detroit Diesel has made a large investment to develop the needed improvements to build the cleanest, longest life and most efficient power system on the market.  Detroit Diesel has designed dependability back into diesel. Increased reliability, and cleaner greener need not require twice the fuel burn of propane and gasoline and resulting CO2 as some would suggest. The Detroit DD5 engine is showing how to lower fleets TCO by affordably and reliably operating an efficient super clean fleet.

Printable Version