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Dyno Testing of Enova Systems Plug-In Hybrid School Bus Shows 70% Improved Fuel Economy and Significantly Reduced Emissions

Recent dynamometer test results show that IC Bus school buses using post-transmission plug-in hybrid drive systems from Enova Systems (earlier post) can improve fuel efficiency by more than 70% compared to standard diesel-powered school buses. The Enova hybrid drive systems installed in the IC Bus school buses also significantly reduce carbon dioxide, NOx and particulate matter emissions.

The series of fuel consumption and emissions tests were conducted to simulate actual route cycles and conditions using the West Virginia University Suburban Test Cycle. The tests were conducted at an independent commercial laboratory accepted by the California Air Resources Board that is properly equipped to perform specialized tests in accordance with applicable federal and California test procedures.

Tests were conducted on the following buses equipped with the Enova plug-in hybrid electric (charge depleting) systems:

  • 2007 model year IC Bus CE Series school bus with a VT365 engine and Allison 2500 transmission

  • 2008 model year IC Bus CE Series school bus with a MaxxForce 7 engine and Allison 2500 transmission

Compared to identical IC Bus school buses with standard diesel engines and drive systems, the test of both plug-in buses showed:

  • More than 70% increase in fuel economy over the standard diesel system;

  • More than 40% reduction in CO2;

  • More than 30% reduction in Particulate Matter; and

  • More than 20% reduction in NOx.

The results are based on the route-specific conditions defined by the West Virginia University Suburban Cycle and do not necessarily reflect fuel consumption and emissions data that may be generated from other route conditions. Selecting routes with frequent stops and starts and minimal highway miles allows the bus to get the most out of the hybrid system and is a key element to obtaining similar results.

Enova’s post transmission parallel hybrid system is one in which the electric drive system is integrated behind the factory-installed transmission. The Enova Hybrid has proven to be a non-invasive system that requires little or no modification to OEM chassis, body or instrumentation. The hybrid systems are designed to communicate with many other electronic systems on the bus, such as engine, transmissions, emissions and ABS systems controllers.

The initial Enova Charge Depleting 25/80-kW hybrid-electric powertrains incorporate a 22.5 kWh lithium-ion battery pack.

The plug-in bus uses a blended operating strategy and in charge depletion mode draws the pack down to approximately a 25% state of charge. Subsequent to being drawn down, the vehicle will switch to charge sustaining mode, with a =20% to 50% improvement in fuel economy for the rest of the route. Charge depletion occurs over the first 40 miles of the route. It is also tailorable, allowing the system to draw down quicker and provide greater mpg improvement or a sorter amount of miles, or drawn down more slowly resulting in a lower mpg improvement, but over a greater mileage range. The depleting cycle is a direct result of the software programming within the Hybrid Control Unit.

We are very pleased with the test results, and will now focus our attention to understanding school bus routes and determining the types and duration of routes that can strongly benefit from plug-in hybrid electric systems. This proves that significant benefits are available as long as we put these in the right place,

—Ewan Pritchard, Hybrid Program Manager for Advanced Energy

In March 2007, the first plug-in hybrid electric school bus was delivered to Manatee County Schools in Bradenton, FL., by IC Bus, Enova Systems and Advanced Energy, a Raleigh, N.C.-based non-profit focused on market-based approaches to energy issues. Today, 19 hybrid school buses are on the road in 11 states due to the efforts of Advanced Energy and its partners.

We believe these results show the success of our system in obtaining fuel economy and air quality improvements without major changes to standard production engines. But this is only the first phase of development. IC Bus and Enova are committed to developing ‘engine off’ technology to obtain additional fuel economy and emissions control improvements.

—David Hillman, Marketing Director for IC Bus

IC Bus recently announced that it will reduce the price of their school buses equipped with plug-in hybrid electric systems by up to $40,000.



The website of the IC-Corp is very informative on this issue as well.


I wonder how much reduction in noise this system achieves. Compared with EU diesel busses, US school busses are literally "noise propelled" - being at least twice or three times as noisy as european busses... (Makes me wonder if US school kids not get deaf over the years of having to rely on such modes of transportation... :)

s dogood

I wonder if a serial hybrid would produce even greater results. Or if a hydraulic hybrid is a financially more feasible approach for school buses. Hard to imagine too many schools are going to bus their kids with expensive lithium ion batteries only to have class in unheated/cooled trailers.


I have thought of retrofits for vehicles like this. Pickups and SUVs with a motor between the transmission and drive shaft. I came to the conclusion that not enough people would do the upgrade.

I am glad to see that they are doing this here. All you have to do is be behind a school bus when it pulls away from the curb to know that this is a great idea. Most buses roar off leaving a black plume of smelly smoke. This is MUCH better.



Harvey D

This is very good news.

A massive school bus conversion program could be financed by the School Commissions + Cities/Counties + States/Provinces + Fed Goverments. Up the fuel tax by 1 cent/Litre if you have to, but let's get it done NOW. The technology is there, let's use it.

We live too close to a Regional Secondary School and the high noise and black smoke from the 30-50 buses is certainly not very healthy.

We would not mind an extra 50-60 cents a week for fuel to fix that old problem.


Have to agree w/ Dogood. I heard another report yesterday about how one of the school systems in my area is getting short changed by the county. Granted the county, schoolwide, is doing better than most in this area there is far to go before it will really be adequate.

I don't even have kids yet, and this has just always been an issue on my mind.


California in 1978 passed Proposition 13 which capped property taxes at 1978 levels. The house that sold in 1978 might pay $1000 per year in taxes. The same house sold in 2008 might pay $8000 per year. There are more than 1 million houses that have their property taxes frozen at 1% of assessed values for 30 years, even though there has been inflation and population increases.

The state probably loses at least $4 billion a year just on residential property tax. I am not saying that house should have to pay $8000, but an increase from $1000 to $4000 in 30 years is not too much to ask.

This is why California faces a multi billion dollar deficit this year, because Arnold refuses to rewrite Prop 13, even though his friend Warren Buffet advised him to do this years ago. It is no wonder schools are in bad shape, no one wants to pay taxes. It is fine for you to help me put my kids through school to don't ask me to put your through school...what a country.

Ewan Pritchard

Good summary of the project. Just a note - these buses have a 35 kWh battery pack - 22.5 kWh is the approximate depeletion per plug-in.

Harvey D


The price of lithium battery packs will go down from about $2000 KWh in 2005 to as little as $200 KWh in 2015 (for a BYD China pack). Secondly, they will weight between 1/2 and 1/3 as much, recharge faster and last 3 times longer.

What is too expensive today, will be much more affordable in 5 to 7 years.
A 35 KWh pack price will go from $70K to only $7K. Of course, the very first few thousand units will cost more. That is true with most new products.

I paid $59 for an extra lithium battery for my camera 5 years ago. Last month I bought two similar batteries (with 50% more capacity) for $5.95 each. That's what will happen to vehicle lithium battery packs when they are mass produced, specially in countries with lower labour cost.


Problem is: lithium is a very scarce resources; prices are constantly going up and will do so even more in the future because the reserves are so small. Lithium batteries might be a non-starter.


"Problem is: lithium is a very scarce resources;"

That's just plain wrong! lithium is not scarce.


Forget peak oil. Are we facing peak lithium?


Peak Lithium, not according to the geologist in this article.



Then you read further along in the comments (from your link) and find the link to Saft batteries and see that the typical Lithium content of a 5AH 3.75V battery is ~1.6g which means 53.3 grams for 1kW-hr of batteries.

GM Volt (32 mile range on US06 test) uses 16kW-hr. 853g of Lithium for one volt.

235 million registered vehicles in the US (2006) [excludes freight trucks, buses and motorcycles].

We would need 200 billion grams of Lithium to change out every single registered vehicle in the US to a Volt with a 16kWh battery (with potential of 40mile range and more realistic range of 32 miles).

Worldwide reserves of 3.7Tg (3.7 trillion grams) (reserves base of 10Tg (10 trillion grams)).

The US (with probably the largest fleet of vehicles in the world) will use up 5% of the easily recoverable reserves leaving plenty of capacity for the annual 20 million grams of current Lithium production going towards ceramics, consumer lithium batteries, etc.

Doesn't look like a shortage to me.



I like the programmable controller which lets you trade off the PHEV mileage benefit during charge depletion mode for how far you can travel before having to go into charge sustaining mode.

I would think that this would be beneficial for all PHEVs. Just program the car for the distance you expect to travel before you can plug it in again.


Not to mention that the LI can probably be easily recycled, so once we've mined it we should be able to use it a bit.


I would say the price of lithium might rise however. That could put resistance on the price declines that people hope for. Remember ethanol, demand for corn rises, price for corn rises, profit margins shrink. In this case demand for lithium rises, price for lithium rises and the decline in the price of batteries at the wholesale level slows.

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